Set Theory Concepts and Venn Diagrams

30 questions

Question 1Question

A museum surveyed 150150 visitors regarding their attendance at two special exhibitions: a Fine Art exhibit and a Natural History exhibit. Among the visitors surveyed, 8585 attended the Fine Art exhibit, 7070 attended the Natural History exhibit, and 2020 attended neither exhibit. How many visitors attended both the Fine Art exhibit and the Natural History exhibit?

Show answer & explanation

Answer: 2525

Answer

The correct answer is 2525 visitors.
Using the inclusion-exclusion formula Total=A+BAB+Neither\text{Total} = |A| + |B| - |A \cap B| + \text{Neither}, we substitute the given values: 150=85+70AB+20150 = 85 + 70 - |A \cap B| + 20. Simplifying gives 150=175AB150 = 175 - |A \cap B|, which yields AB=25|A \cap B| = 25. Thus, 2525 visitors attended both exhibits.

Step-by-Step Solution

1
Identify the given set values and formula.
Total visitors =150= 150, Fine Art attendees A=85|A| = 85, Natural History attendees H=70|H| = 70, Neither =20= 20.
The principle of inclusion-exclusion for two sets states that Total=A+HAH+Neither\text{Total} = |A| + |H| - |A \cap H| + \text{Neither}.
2
Calculate the number of visitors who attended at least one exhibit.
AH=15020=130|A \cup H| = 150 - 20 = 130.
Subtracting those who attended neither exhibit from the total population yields the total number of unique visitors who attended at least one of the two exhibits.
3
Solve for the intersection AH|A \cap H|.
130=85+70AH    130=155AH    AH=25130 = 85 + 70 - |A \cap H| \implies 130 = 155 - |A \cap H| \implies |A \cap H| = 25.
Subtracting the union AH|A \cup H| from the sum of the individual sets A+H|A| + |H| eliminates the double-counted intersection.

Key Concept

Two-Set Principle of Inclusion-Exclusion
Estimated Time:1m 0s
Question 2Question

A survey of 120120 university students evaluated course enrollment in Computer Science and Mathematics. Of the students surveyed, 7070 are enrolled in Computer Science, 6565 are enrolled in Mathematics, and 2525 are enrolled in neither course. How many students are enrolled in both Computer Science and Mathematics?

Show answer & explanation

Answer: 40

Answer

40 students are enrolled in both Computer Science and Mathematics.
The correct answer is 40. From the total population of 120 students, 25 take neither subject, meaning 12025=95120 - 25 = 95 students take at least one subject. According to the Principle of Inclusion-Exclusion, CM=C+MCM|C \cup M| = |C| + |M| - |C \cap M|. Substituting the known values yields 95=70+65CM95 = 70 + 65 - |C \cap M|, which simplifies to 95=135CM95 = 135 - |C \cap M|, giving CM=40|C \cap M| = 40.

Step-by-Step Solution

1
Calculate the total number of students enrolled in at least one of the two courses.
Total in at least one course = 12025=95120 - 25 = 95.
Subtracting the students enrolled in neither course from the total surveyed gives the union of the two sets, CM|C \cup M|.
2
Apply the Principle of Inclusion-Exclusion for two sets.
CM=C+MCM|C \cup M| = |C| + |M| - |C \cap M|, so 95=70+65CM95 = 70 + 65 - |C \cap M|.
Adding C|C| and M|M| double-counts the students taking both courses, so subtracting the union yields the intersection.
3
Solve for the intersection CM|C \cap M|.
CM=13595=40|C \cap M| = 135 - 95 = 40.
Direct arithmetic evaluation gives the required count of students enrolled in both subjects.

Key Concept

Principle of Inclusion-Exclusion for Two Sets
Estimated Time:1m 0s
Question 3Question

In a department of 9090 employees, 5555 speak French, 4545 speak Spanish, and 1010 speak neither French nor Spanish. How many employees speak both French and Spanish?

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Answer: 20

Answer

20
Subtracting the 10 employees who speak neither language from the total of 90 leaves 80 employees who speak at least one language. By the principle of inclusion-exclusion, Total(At least one) = French + Spanish - Both. Substituting the known values gives 80 = 55 + 45 - Both, which simplifies to 80 = 100 - Both, so Both = 20.

Step-by-Step Solution

1
Find the number of employees who speak at least one of the languages
80 employees
Subtract the 10 employees who speak neither language from the total department size of 90.
2
Sum the total counts for each language group
100
Add the number of French speakers (55) to Spanish speakers (45).
3
Calculate the overlap (intersection) using inclusion-exclusion
20 employees
Subtract the count of employees speaking at least one language (80) from the sum of the individual language groups (100).

Key Concept

Principle of Inclusion-Exclusion for Two Sets
Estimated Time:45s
Question 4Question

A survey of 200200 financial executives analyzed their personal investment portfolios across three asset classes: Equities (EE), Real Estate (RR), and Commodities (CC). Exactly 2020 executives hold no investments in any of these three asset classes. Among the remaining executives, 115115 hold Equities, 9595 hold Real Estate, and 8080 hold Commodities. Furthermore, 4545 executives hold both Equities and Real Estate, 3535 hold both Real Estate and Commodities, and 4040 hold both Equities and Commodities. How many of the surveyed executives hold investments in exactly two of these three asset classes?

Show answer & explanation

Answer: 90

Answer

90
To find the number of executives holding investments in exactly two asset classes, we first determine the total number of executives invested in at least one asset class: 20020=180200 - 20 = 180. Applying the three-set inclusion-exclusion formula, 180=115+95+80(45+35+40)+ERC180 = 115 + 95 + 80 - (45 + 35 + 40) + |E \cap R \cap C|, which simplifies to 180=170+ERC180 = 170 + |E \cap R \cap C|, giving ERC=10|E \cap R \cap C| = 10. The number of executives holding exactly two asset classes is calculated by subtracting the triple intersection (10) from each pairwise overlap: (4510)+(3510)+(4010)=35+25+30=90(45 - 10) + (35 - 10) + (40 - 10) = 35 + 25 + 30 = 90.

Step-by-Step Solution

1
Calculate the total number of executives holding at least one asset class.
The total number in the union ERC=20020=180|E \cup R \cup C| = 200 - 20 = 180.
Subtracting executives holding no investments gives the cardinality of the union.
2
Use the Principle of Inclusion-Exclusion for three sets to find the triple intersection ERC|E \cap R \cap C|.
ERC=E+R+C(ER+RC+EC)+ERC|E \cup R \cup C| = |E| + |R| + |C| - (|E \cap R| + |R \cap C| + |E \cap C|) + |E \cap R \cap C|, so 180=115+95+80(45+35+40)+ERC180 = 115 + 95 + 80 - (45 + 35 + 40) + |E \cap R \cap C|, yielding 180=290120+ERC=170+ERC180 = 290 - 120 + |E \cap R \cap C| = 170 + |E \cap R \cap C|, hence ERC=10|E \cap R \cap C| = 10.
The inclusion-exclusion formula relates the union, individual set sizes, pairwise overlaps, and the triple overlap.
3
Determine the number of executives in each pairwise intersection that belong ONLY to those two sets.
Equities and Real Estate only: 4510=3545 - 10 = 35; Real Estate and Commodities only: 3510=2535 - 10 = 25; Equities and Commodities only: 4010=3040 - 10 = 30.
Subtracting the triple intersection count from each pairwise intersection isolates those holding exactly two asset classes.
4
Sum the counts for executives holding exactly two asset classes.
35+25+30=9035 + 25 + 30 = 90.
Adding these disjoint sets yields the total number of executives in exactly two categories.

Key Concept

Three-Set Inclusion-Exclusion Principle and Venn Diagram Region Decomposition
Estimated Time:2m 0s
Question 5Question

An agricultural research station evaluated 300300 soil plots for the presence of three specific mineral deficiencies: Nitrogen (NN), Phosphorus (PP), and Potassium (KK). The survey revealed the following data:
- Exactly 6060 plots exhibited none of the three deficiencies.
- 140140 plots exhibited Nitrogen deficiency (NN).
- 130130 plots exhibited Phosphorus deficiency (PP).
- 120120 plots exhibited Potassium deficiency (KK).
- Exactly 7070 plots exhibited exactly two of the three deficiencies.

Which of the following statements MUST be true? Select all such statements.

Select all that apply

Show answer & explanation

Answer: Exactly 4040 plots exhibited all three mineral deficiencies.; Exactly 130130 plots exhibited exactly one mineral deficiency.; The number of plots exhibiting at least two mineral deficiencies is 110110.

Answer

The statements confirming that exactly 40 plots exhibited all three deficiencies, exactly 130 plots exhibited exactly one deficiency, and 110 plots exhibited at least two deficiencies are all correct.
The system of set equations shows that n3=40n_3 = 40 plots have all three deficiencies, n1=130n_1 = 130 plots have exactly one deficiency, and n2+n3=70+40=110n_2 + n_3 = 70 + 40 = 110 plots have at least two deficiencies. Therefore, the statements asserting 4040 plots for all three deficiencies, 130130 plots for exactly one deficiency, and 110110 plots for at least two deficiencies are all guaranteed to be true.

Step-by-Step Solution

1
Determine the total number of plots exhibiting at least one deficiency.
The total number of plots with at least one deficiency is 30060=240300 - 60 = 240.
Plots with no deficiencies are excluded from the set union NPK|N \cup P \cup K|.
2
Set up the inclusion-exclusion equations for set membership.
Let n1n_1 be the number of plots with exactly one deficiency, n2=70n_2 = 70 be the number of plots with exactly two deficiencies, and n3n_3 be the number of plots with all three deficiencies.
Equation 1 (Total elements in union): n1+n2+n3=240    n1+70+n3=240    n1+n3=170n_1 + n_2 + n_3 = 240 \implies n_1 + 70 + n_3 = 240 \implies n_1 + n_3 = 170.
Equation 2 (Sum of individual set cardinalities): N+P+K=n1+2n2+3n3    140+130+120=n1+2(70)+3n3    390=n1+140+3n3    n1+3n3=250|N| + |P| + |K| = n_1 + 2n_2 + 3n_3 \implies 140 + 130 + 120 = n_1 + 2(70) + 3n_3 \implies 390 = n_1 + 140 + 3n_3 \implies n_1 + 3n_3 = 250.
Each element in an individual set sum is counted once for single-set membership, twice for double-set membership, and three times for triple-set membership.
3
Solve the system of linear equations for n1n_1 and n3n_3.
Subtract Equation 1 from Equation 2: (n1+3n3)(n1+n3)=250170    2n3=80    n3=40(n_1 + 3n_3) - (n_1 + n_3) = 250 - 170 \implies 2n_3 = 80 \implies n_3 = 40.
Substitute n3=40n_3 = 40 into Equation 1: n1+40=170    n1=130n_1 + 40 = 170 \implies n_1 = 130.
This yields 4040 plots with all three deficiencies and 130130 plots with exactly one deficiency.
4
Evaluate each option statement against the computed set cardinalities.
1) All three deficiencies (n3n_3) = 4040 (True).
2) Exactly one deficiency (n1n_1) = 130130 (True).
3) At least two deficiencies (n2+n3n_2 + n_3) = 70+40=11070 + 40 = 110 (True).
4) At most two deficiencies = Total - n3=30040=260170n_3 = 300 - 40 = 260 \neq 170 (False).
5) Nitrogen deficiency only depends on how the 7070 dual-deficiency plots are split between NPN \cap P, NKN \cap K, and PKP \cap K, which is not uniquely determined by the given data (False).
Only statements meeting the exact numerical bounds and logical guarantees must be true.

Key Concept

Three-Set Principle of Inclusion-Exclusion and Partitioning into Disjoint Regions
Question 6Question

An agricultural research station evaluated a sample of 120120 organic farm plots for contamination by three types of crop fungi: *Fusarium* (FF), *Rhizoctonia* (RR), and *Sclerotinia* (SS). The audit revealed the following data:

- 5555 plots were contaminated with *Fusarium*.
- 5050 plots were contaminated with *Rhizoctonia*.
- 4545 plots were contaminated with *Sclerotinia*.
- 2020 plots were contaminated with both *Fusarium* and *Rhizoctonia*.
- 1515 plots were contaminated with both *Rhizoctonia* and *Sclerotinia*.
- 1818 plots were contaminated with both *Fusarium* and *Sclerotinia*.
- 88 plots were contaminated with all three fungi.

Which of the following statements must be true? Select all such statements.

Select all that apply

Show answer & explanation

Answer: Exactly 15 farm plots were free from all three types of crop fungi.; Exactly 29 farm plots were contaminated by exactly two of the three crop fungi.

Answer

The correct statements are that exactly 15 farm plots were free from all three types of crop fungi, and exactly 29 farm plots were contaminated by exactly two of the three crop fungi.
The statement regarding 15 farm plots being free from all three fungi is correct because the total number of plots contaminated by at least one fungus is calculated using inclusion-exclusion: 55 + 50 + 45 - 20 - 15 - 18 + 8 = 105, leaving 120 - 105 = 15 plots clean. The statement regarding 29 farm plots contaminated by exactly two fungi is correct because summing the mutually exclusive regions containing exactly two fungi yields (20 - 8) + (15 - 8) + (18 - 8) = 12 + 7 + 10 = 29.

Step-by-Step Solution

1
Calculate the non-overlapping regions for each intersection.
Triple intersection |F ∩ R ∩ S| = 8.
Region |F ∩ R only| = 20 - 8 = 12.
Region |R ∩ S only| = 15 - 8 = 7.
Region |F ∩ S only| = 18 - 8 = 10.
Determining exact two-set regions requires removing elements that belong to all three sets.
2
Calculate single-fungus-only regions.
|F only| = 55 - (12 + 10 + 8) = 25.
|R only| = 50 - (12 + 7 + 8) = 23.
|S only| = 45 - (10 + 7 + 8) = 20.
Subtract all double and triple overlaps from total individual set counts.
3
Determine union of all three sets and the neither region.
Total contaminated = 25 + 23 + 20 + 12 + 7 + 10 + 8 = 105.
Plots free from all fungi = 120 - 105 = 15.
Subtracting the total contaminated plots from the overall sample size gives the plots outside all sets.
4
Verify statement validity.
Plots free of all fungi = 15 (Valid).
Plots with exactly two fungi = 12 + 7 + 10 = 29 (Valid).
Match calculated regional counts against the presented options.

Key Concept

Three-Set Principle of Inclusion-Exclusion and Venn Diagram Partitioning
Question 7Question

An urban planning department surveyed a total of 360360 commercial buildings regarding three sustainability certifications: LEED (LL), BREEAM (BB), and WELL (WW). Exactly 5050 of the surveyed buildings held none of the three certifications. The survey revealed the following data:

180180 buildings held the LEED certification.
140140 buildings held the BREEAM certification.
130130 buildings held the WELL certification.
4545 buildings held both the LEED and BREEAM certifications.
4040 buildings held both the BREEAM and WELL certifications.
2525 buildings held all three certifications.

How many of the surveyed buildings held EXACTLY TWO of the three certifications?

Show answer & explanation

Answer: 90

Answer

90
To find the number of buildings with exactly two certifications, we first determine the total number of buildings holding at least one certification: 36050=310360 - 50 = 310. Applying the three-set inclusion-exclusion formula, 310=180+140+130(45+40+LW)+25310 = 180 + 140 + 130 - (45 + 40 + |L \cap W|) + 25, which simplifies to 310=390LW310 = 390 - |L \cap W|, giving LW=80|L \cap W| = 80. Next, we isolate the regions holding exactly two certifications by subtracting the 25 triple-certified buildings from each pairwise intersection: LEED & BREEAM only = 4525=2045 - 25 = 20, BREEAM & WELL only = 4025=1540 - 25 = 15, and LEED & WELL only = 8025=5580 - 25 = 55. Summing these three exclusive regions gives 20+15+55=9020 + 15 + 55 = 90.

Step-by-Step Solution

1
Determine the total size of the union of all three sets
LBW=36050=310|L \cup B \cup W| = 360 - 50 = 310
Buildings holding at least one certification represent the entire surveyed population minus those holding no certifications.
2
Solve for the unknown overlap of LEED and WELL certifications using the inclusion-exclusion formula
LW=80|L \cap W| = 80
Using LBW=L+B+WLBBWLW+LBW|L \cup B \cup W| = |L| + |B| + |W| - |L \cap B| - |B \cap W| - |L \cap W| + |L \cap B \cap W|, we have 310=180+140+1304540LW+25=390LW310 = 180 + 140 + 130 - 45 - 40 - |L \cap W| + 25 = 390 - |L \cap W|.
3
Calculate the number of buildings in each region corresponding to exactly two certifications
LEED and BREEAM only: 4525=2045 - 25 = 20; BREEAM and WELL only: 4025=1540 - 25 = 15; LEED and WELL only: 8025=5580 - 25 = 55
Each total pairwise intersection includes the 25 buildings that hold all three certifications, so subtracting 25 isolates those holding exclusively two certifications.
4
Sum the three isolated regions
20+15+55=9020 + 15 + 55 = 90
The total number of buildings holding exactly two certifications is the sum of the three non-overlapping regions representing two certifications.

Key Concept

Three-Set Inclusion-Exclusion Principle and Partitioning Venn Diagrams
Estimated Time:2m 0s
Question 8Question

A total of 120 software engineers at a firm were surveyed about their attendance at three professional development workshops: Machine Learning (MM), Cloud Computing (CC), and Cybersecurity (SS). The survey results revealed the following:

- 65 engineers attended Machine Learning
- 55 engineers attended Cloud Computing
- 48 engineers attended Cybersecurity
- 25 engineers attended both Machine Learning and Cloud Computing
- 18 engineers attended both Cloud Computing and Cybersecurity
- 20 engineers attended both Machine Learning and Cybersecurity
- 10 engineers attended all three workshops

How many of the surveyed software engineers attended exactly one of the three workshops?

Show answer & explanation

Answer: 72

Answer

72 engineers attended exactly one of the three workshops.
To find the number of engineers attending exactly one workshop, calculate the exclusive single-set counts for each category using a 3-set Venn diagram model. Subtracting the overlapping regions gives 30 for Machine Learning only, 22 for Cloud Computing only, and 20 for Cybersecurity only. Summing these exclusive regions yields 30+22+20=7230 + 22 + 20 = 72. Alternatively, using the algebraic formula Exactly 1=(M+C+S)2(MC+CS+MS)+3MCS=(65+55+48)2(25+18+20)+3(10)=168126+30=72|\text{Exactly 1}| = (|M| + |C| + |S|) - 2(|M \cap C| + |C \cap S| + |M \cap S|) + 3|M \cap C \cap S| = (65 + 55 + 48) - 2(25 + 18 + 20) + 3(10) = 168 - 126 + 30 = 72.

Step-by-Step Solution

1
Find the number of engineers in the regions corresponding to exactly two workshops.
Machine Learning and Cloud Computing only = 2510=1525 - 10 = 15; Cloud Computing and Cybersecurity only = 1810=818 - 10 = 8; Machine Learning and Cybersecurity only = 2010=1020 - 10 = 10.
The given pairwise intersection values include engineers who attended all three workshops.
2
Calculate the number of engineers who attended only one specific workshop.
Machine Learning only = 65(15+10+10)=3065 - (15 + 10 + 10) = 30; Cloud Computing only = 55(15+8+10)=2255 - (15 + 8 + 10) = 22; Cybersecurity only = 48(10+8+10)=2048 - (10 + 8 + 10) = 20.
Subtract all overlapping regions (two-workshop-only and three-workshop) from each set's total.
3
Sum the single-workshop regions to find the total attending exactly one workshop.
30+22+20=7230 + 22 + 20 = 72.
The subsets representing single workshop attendance are disjoint.

Key Concept

Principle of Inclusion-Exclusion for Three Sets and Venn Diagram Region Partitioning
Estimated Time:2m 0s
Question 9Question

A health club surveyed 100100 members regarding their use of cardio machines and weight equipment. Of the members surveyed, 6060 use cardio machines, 5050 use weight equipment, and 1515 use neither cardio machines nor weight equipment. Which of the following statements must be true? Indicate all such statements.

Select all that apply

Show answer & explanation

Answer: Exactly 2525 members use both cardio machines and weight equipment.; Exactly 3535 members use cardio machines but do not use weight equipment.; Exactly 8585 members use at least one of the two types of equipment.

Answer

The true statements are those stating that exactly 25 members use both types of equipment, exactly 35 members use cardio machines only, and exactly 85 members use at least one type of equipment.
The total surveyed population of 100100 consists of members using at least one type of equipment and members using neither (1515). Thus, 10015=85100 - 15 = 85 members use at least one type of equipment. Using the inclusion-exclusion formula Union=Cardio+WeightsIntersection\text{Union} = \text{Cardio} + \text{Weights} - \text{Intersection}, we have 85=60+50Intersection85 = 60 + 50 - \text{Intersection}, giving an intersection of 2525 members using both types of equipment. Subtracting 2525 from the 6060 total cardio users shows that 3535 members use cardio machines exclusively.

Step-by-Step Solution

1
Calculate the total number of members using at least one type of equipment.
Total using at least one type =10015=85= 100 - 15 = 85.
Members using at least one type of equipment represent the total surveyed population minus those who use neither.
2
Calculate the number of members using both cardio machines and weight equipment using the principle of inclusion-exclusion.
Both=Cardio+WeightsAt least one=60+5085=25\text{Both} = \text{Cardio} + \text{Weights} - \text{At least one} = 60 + 50 - 85 = 25.
Adding the individual set counts double-counts the overlapping region, so subtracting the total union gives the intersection size.
3
Calculate the number of members using cardio machines only.
Cardio Only=CardioBoth=6025=35\text{Cardio Only} = \text{Cardio} - \text{Both} = 60 - 25 = 35.
Subtracting the intersection from the total cardio group yields the exclusive cardio users.

Key Concept

Principle of Inclusion-Exclusion for Two Sets
Estimated Time:1m 15s
Question 10Question

A survey of 250250 registered voters was conducted to analyze their primary news sources: Television (TT), the Internet (II), and Print newspapers (PP). The survey revealed the following results:

- 140140 voters get news from Television.
- 150150 voters get news from the Internet.
- 8080 voters get news from Print newspapers.
- 4545 voters get news from both Television and Print newspapers.
- 6060 voters get news from both the Internet and Print newspapers.
- 3030 voters get news from all three sources.
- 2020 voters do not get news from any of these three sources.

How many of the surveyed voters get news from Television and the Internet, but NOT from Print newspapers?

Show answer & explanation

Answer: 35

Answer

35 voters get news from Television and the Internet, but not from Print newspapers.
Using the principle of inclusion-exclusion for three sets, the total union size is 230 voters (250 total minus 20 who use none). Setting up the formula 230 = 140 + 150 + 80 - 45 - 60 - |T ∩ I| + 30 allows us to solve for |T ∩ I| = 65. To find those who use Television and Internet but NOT Print newspapers, we subtract the 30 voters who use all three sources from 65, resulting in 35 voters.

Step-by-Step Solution

1
Find the size of the union of all three sets
|T ∪ I ∪ P| = 250 - 20 = 230
Subtracting the 20 voters who use none of the three news sources from the total sample of 250 gives the total number of voters in at least one category.
2
Set up the Principle of Inclusion-Exclusion for three sets
230 = 140 + 150 + 80 - 45 - 60 - |T ∩ I| + 30
The formula sums individual set sizes, subtracts pairwise intersections, and adds back the triple intersection.
3
Solve for the total intersection of Television and Internet
|T ∩ I| = 65
Simplifying the equation gives 230 = 295 - |T ∩ I|, which yields |T ∩ I| = 65.
4
Exclude those who also read Print newspapers
|(T ∩ I) \ P| = 65 - 30 = 35
Subtracting the 30 voters who use all three sources leaves only those who use Television and Internet without Print newspapers.

Key Concept

Three-Set Principle of Inclusion-Exclusion
Question 11Question

A market research firm surveyed 150150 venture capital investors regarding their investments in three emerging sectors: Artificial Intelligence (AA), Biotechnology (BB), and Clean Energy (CC). The survey revealed the following data:

- 1515 investors did not invest in any of the three sectors.
- 7878 investors invested in Artificial Intelligence.
- 6565 investors invested in Biotechnology.
- 6262 investors invested in Clean Energy.
- 1010 investors invested in all three sectors.

If 5050 investors invested in exactly two of the three sectors, how many investors invested in exactly one of the three sectors?

Show answer & explanation

Answer: 75

Answer

75 investors invested in exactly one of the three sectors.
The total number of investors who invested in at least one of the three sectors is 15015=135150 - 15 = 135. In a 3-set Venn diagram, the total union is the sum of those who belong to exactly one sector (E1E_1), exactly two sectors (E2E_2), and all three sectors (E3E_3). We are given E2=50E_2 = 50 and E3=10E_3 = 10. Setting up the region equation E1+E2+E3=135E_1 + E_2 + E_3 = 135 gives E1+50+10=135E_1 + 50 + 10 = 135, which yields E1=75E_1 = 75.

Step-by-Step Solution

1
Calculate the total number of investors who invested in at least one sector (the union of sets A, B, and C).
ABC=15015=135|A \cup B \cup C| = 150 - 15 = 135
Subtracting the investors who invested in none of the sectors from the total population yields the size of the union.
2
Express the union as the sum of non-overlapping region counts.
ABC=E1+E2+E3|A \cup B \cup C| = E_1 + E_2 + E_3, where E1E_1 is the number of investors in exactly 1 sector, E2E_2 is the number in exactly 2 sectors, and E3E_3 is the number in all 3 sectors.
Any element in the union belongs to exactly one, exactly two, or all three sets.
3
Substitute the known values into the partition equation and solve for E1E_1.
135=E1+50+10    E1=13560=75135 = E_1 + 50 + 10 \implies E_1 = 135 - 60 = 75
We are given E2=50E_2 = 50 and E3=10E_3 = 10, leaving E1E_1 as the remaining unknown value.

Key Concept

Three-Set Inclusion-Exclusion and Region Partitioning
Estimated Time:2m 0s
Question 12Question

A specialty coffee roastery surveyed 200200 customers to determine their preferences among three origin beans: Ethiopian (EE), Colombian (CC), and Guatemalan (GG). Every customer surveyed preferred at least one of the three origins. The survey revealed that 105105 customers preferred Ethiopian, 9090 preferred Colombian, and 8585 preferred Guatemalan. Additionally, 3535 customers preferred both Ethiopian and Colombian, 3030 preferred both Colombian and Guatemalan, and 4040 preferred both Ethiopian and Guatemalan. How many customers preferred all three origin beans?

Show answer & explanation

Answer: 2525

Answer

25 customers preferred all three origin beans.
The correct answer is obtained by setting up the three-set inclusion-exclusion formula ABC=A+B+C(AB+BC+AC)+ABC|A \cup B \cup C| = |A| + |B| + |C| - (|A \cap B| + |B \cap C| + |A \cap C|) + |A \cap B \cap C|. Substituting the given values gives 200=105+90+85(35+30+40)+x200 = 105 + 90 + 85 - (35 + 30 + 40) + x. Solving 200=175+x200 = 175 + x yields x=25x = 25.

Step-by-Step Solution

1
State the 3-set Inclusion-Exclusion Principle formula.
ECG=E+C+G(EC+CG+EG)+ECG|E \cup C \cup G| = |E| + |C| + |G| - (|E \cap C| + |C \cap G| + |E \cap G|) + |E \cap C \cap G|
This fundamental formula relates the total union of three overlapping sets to their individual sizes, pairwise intersections, and three-set intersection.
2
Substitute the given values into the formula.
200=105+90+85(35+30+40)+x200 = 105 + 90 + 85 - (35 + 30 + 40) + x, where x=ECGx = |E \cap C \cap G|
Since every customer preferred at least one origin, the union of all three sets equals the total number of customers surveyed (200200).
3
Simplify the sum of individual sets and pairwise intersections.
Sum of individual sets = 105+90+85=280105 + 90 + 85 = 280. Sum of pairwise intersections = 35+30+40=10535 + 30 + 40 = 105.
Grouping terms makes evaluating the equation easier and reduces calculation errors.
4
Solve for the unknown three-set intersection xx.
200=280105+x    200=175+x    x=25200 = 280 - 105 + x \implies 200 = 175 + x \implies x = 25
Subtracting 175175 from 200200 yields the number of customers who prefer all three origin beans.

Key Concept

Three-Set Principle of Inclusion-Exclusion
Question 13Question

An aviation safety committee audited 240240 international flights for operational delays caused by Weather (WW), Mechanical issues (MM), and Air Traffic Control (TT). The audit revealed the following data:

- 112112 flights experienced Weather delays.
- 9898 flights experienced Mechanical delays.
- 104104 flights experienced Air Traffic Control delays.
- 2828 flights experienced both Weather and Mechanical delays.
- 3434 flights experienced both Mechanical and Air Traffic Control delays.
- 4242 flights experienced both Weather and Air Traffic Control delays.
- The number of flights that experienced none of these three types of delays was equal to twice the number of flights that experienced all three types of delays.

How many flights experienced exactly one of these three types of delays?

Show answer & explanation

Answer: 136

Answer

The number of flights that experienced exactly one of the three types of delays is 136.
The total number of audited flights is partitioned into those experiencing at least one delay and those experiencing none. By applying the Inclusion-Exclusion formula WMT=112+98+104(28+34+42)+x=210+x|W \cup M \cup T| = 112 + 98 + 104 - (28 + 34 + 42) + x = 210 + x, and setting the total audited flights to 240=(210+x)+2x240 = (210 + x) + 2x, we solve for the triple intersection x=10x = 10. Isolating the exclusive single-delay regions yields 5252 flights for Weather only, 4646 for Mechanical only, and 3838 for ATC only, giving a total of 136136 flights.

Step-by-Step Solution

1
Define the unknown intersection and external set quantities in terms of a single variable.
Let x=WMTx = |W \cap M \cap T| represent the number of flights experiencing all three types of delays. Consequently, the number of flights experiencing none of the delays is 2x2x.
Relating the unknown triple intersection to the 'neither' category allows all regions to be expressed in terms of xx.
2
Apply the Principle of Inclusion-Exclusion for three overlapping sets to find the total number of flights with at least one delay.
WMT=W+M+T(WM+MT+WT)+WMT=112+98+104(28+34+42)+x=210+x|W \cup M \cup T| = |W| + |M| + |T| - (|W \cap M| + |M \cap T| + |W \cap T|) + |W \cap M \cap T| = 112 + 98 + 104 - (28 + 34 + 42) + x = 210 + x.
Subtracting pairwise intersections corrects for double-counting, and adding back the triple intersection corrects for over-subtraction.
3
Formulate and solve the linear equation for the total sample population of 240 flights.
Total Flights=WMT+Neither    240=(210+x)+2x    240=210+3x    3x=30    x=10\text{Total Flights} = |W \cup M \cup T| + |\text{Neither}| \implies 240 = (210 + x) + 2x \implies 240 = 210 + 3x \implies 3x = 30 \implies x = 10.
The sum of flights in the union of the sets and flights outside all three sets must equal the total number of flights audited.
4
Determine the number of flights in the two-set intersection regions excluding the triple intersection.
Weather and Mechanical only: 2810=1828 - 10 = 18; Mechanical and ATC only: 3410=2434 - 10 = 24; Weather and ATC only: 4210=3242 - 10 = 32.
Each pairwise intersection given in the stem includes the triple intersection of 10 flights.
5
Compute the number of flights belonging exclusively to single categories and sum them.
Weather only: 112(18+32+10)=52112 - (18 + 32 + 10) = 52; Mechanical only: 98(18+24+10)=4698 - (18 + 24 + 10) = 46; ATC only: 104(32+24+10)=38104 - (32 + 24 + 10) = 38. Sum of single categories: 52+46+38=13652 + 46 + 38 = 136.
Subtracting all overlapping components from each set total isolates the exclusive single-category elements.

Key Concept

Three-Set Inclusion-Exclusion Principle and Venn Diagram Region Partitioning
Estimated Time:2m 30s
Question 14Question

A technology firm surveyed 100100 software developers regarding their proficiency in three programming languages: Python (PP), Java (JJ), and C++ (CC). The survey revealed the following results:
- 5555 developers are proficient in Python.
- 5050 developers are proficient in Java.
- 3535 developers are proficient in C++.
- 2525 developers are proficient in both Python and Java.
- 2020 developers are proficient in both Java and C++.
- 1515 developers are proficient in both Python and C++.
- 1010 developers are proficient in none of these three languages.

Which of the following statements must be true? Select all such statements.

Select all that apply

Show answer & explanation

Answer: Exactly 1010 developers are proficient in all three programming languages.; The number of developers proficient in Python only is 2525.; The total number of developers proficient in exactly one of these languages is 5050.

Answer

The correct statements are those asserting that exactly 10 developers know all three languages, that 25 developers know Python only, and that 50 developers know exactly one of these languages.
The statements confirming that 10 developers know all three languages, 25 know Python only, and 50 know exactly one language are correct based on standard 3-set Venn diagram calculations. Using Inclusion-Exclusion, PJC=90|P \cup J \cup C| = 90, which gives PJC=10|P \cap J \cap C| = 10. Subtracting overlapping regions yields 2525 for Python only, 1515 for Java only, 1010 for C++ only, summing to 5050 for exactly one language.

Step-by-Step Solution

1
Calculate the total number of developers proficient in at least one language.
Nat least one=10010=90N_{\text{at least one}} = 100 - 10 = 90.
Subtracting the developers proficient in none of the languages from the total survey population yields the union PJC|P \cup J \cup C|.
2
Apply the Principle of Inclusion-Exclusion for 3 sets to find the triple intersection PJC|P \cap J \cap C|.
90=55+50+35(25+20+15)+PJC    90=80+PJC    PJC=1090 = 55 + 50 + 35 - (25 + 20 + 15) + |P \cap J \cap C| \implies 90 = 80 + |P \cap J \cap C| \implies |P \cap J \cap C| = 10.
The formula PJC=P+J+C(PJ+JC+PC)+PJC|P \cup J \cup C| = |P| + |J| + |C| - (|P \cap J| + |J \cap C| + |P \cap C|) + |P \cap J \cap C| links all given set quantities.
3
Calculate the count of developers proficient in exactly two languages for each pair.
Python and Java only = 2510=1525 - 10 = 15; Java and C++ only = 2010=1020 - 10 = 10; Python and C++ only = 1510=515 - 10 = 5.
Subtracting the triple intersection count (1010) from each pairwise intersection yields the exclusive two-set regions.
4
Calculate the single-language proficiency counts (exactly one language).
Python only = 55(15+5+10)=2555 - (15 + 5 + 10) = 25; Java only = 50(15+10+10)=1550 - (15 + 10 + 10) = 15; C++ only = 35(5+10+10)=1035 - (5 + 10 + 10) = 10. Total exactly one = 25+15+10=5025 + 15 + 10 = 50.
Subtracting all overlapping regions containing each language from its total count yields the single-language region size.
5
Evaluate the statement options based on computed regional values.
Statements stating 10 all three, 25 Python only, and 50 exactly one language are true.
Comparing calculated values (1010 for all three, 2525 for Python only, 5050 for exactly one language) against each option validates the true choices.

Key Concept

Three-set inclusion-exclusion principle and Venn diagram region decomposition
Question 15Question

A technology institute surveyed a cohort of 300300 software engineers regarding their proficiency in three programming paradigms: Functional (FF), Object-Oriented (OO), and Concurrent (CC). Every surveyed engineer is proficient in at least one of these three paradigms. The ratio of the total number of engineers proficient in FF, OO, and CC is 5:6:45 : 6 : 4, respectively. Furthermore, exactly 20%20\% of the engineers proficient in FF are proficient in all three paradigms. If exactly 5454 engineers are proficient in both FF and OO, 4848 are proficient in both OO and CC, and 3030 are proficient in both FF and CC, how many engineers in the cohort are proficient in exactly one programming paradigm?

Show answer & explanation

Answer: 222

Answer

222 engineers are proficient in exactly one programming paradigm.
Using the Principle of Inclusion-Exclusion for three sets, FOC=F+O+C(FO+OC+FC)+FOC|F \cup O \cup C| = |F| + |O| + |C| - (|F \cap O| + |O \cap C| + |F \cap C|) + |F \cap O \cap C|. Substituting F=5k|F|=5k, O=6k|O|=6k, C=4k|C|=4k, FOC=k|F \cap O \cap C|=k, and the total cohort 300300 gives 300=16k132300 = 16k - 132, yielding k=27k = 27. Consequently, the triple intersection is 2727. Decomposing into disjoint regions: FF only =78= 78, OO only =87= 87, and CC only =57= 57. Summing these gives 78+87+57=22278 + 87 + 57 = 222.

Step-by-Step Solution

1
Set up algebraic representations for the set sizes using the given ratio.
Let F=5k|F| = 5k, O=6k|O| = 6k, and C=4k|C| = 4k for some positive constant kk.
The total proficiencies follow the ratio 5:6:45:6:4.
2
Express the triple intersection FOC|F \cap O \cap C| in terms of kk.
FOC=0.20×F=0.20×5k=k|F \cap O \cap C| = 0.20 \times |F| = 0.20 \times 5k = k.
Exactly 20% of engineers proficient in FF are proficient in all three paradigms.
3
Apply the Principle of Inclusion-Exclusion for three sets to solve for kk.
FOC=F+O+C(FO+OC+FC)+FOC    300=5k+6k+4k(54+48+30)+k    300=16k132    16k=432    k=27|F \cup O \cup C| = |F| + |O| + |C| - (|F \cap O| + |O \cap C| + |F \cap C|) + |F \cap O \cap C| \implies 300 = 5k + 6k + 4k - (54 + 48 + 30) + k \implies 300 = 16k - 132 \implies 16k = 432 \implies k = 27.
Every engineer is proficient in at least one paradigm, so FOC=300|F \cup O \cap C| = 300.
4
Calculate the total size of each set and each exclusive intersection region.
F=135|F| = 135, O=162|O| = 162, C=108|C| = 108, and FOC=27|F \cap O \cap C| = 27.
Exclusively FO=5427=27F \cap O = 54 - 27 = 27.
Exclusively OC=4827=21O \cap C = 48 - 27 = 21.
Exclusively FC=3027=3F \cap C = 30 - 27 = 3.
Subtracting the triple intersection from pairwise intersections yields the two-set-only regions.
5
Determine the number of engineers proficient in exactly one paradigm.
Only F=135(27+3+27)=78F = 135 - (27 + 3 + 27) = 78.
Only O=162(27+21+27)=87O = 162 - (27 + 21 + 27) = 87.
Only C=108(3+21+27)=57C = 108 - (3 + 21 + 27) = 57.
Total exactly one = 78+87+57=22278 + 87 + 57 = 222.
Subtracting all overlapping regions from each total set size gives the single-category populations.

Key Concept

Three-set Principle of Inclusion-Exclusion and Venn Diagram region decomposition.
Question 16Question

A survey of 300300 urban commuters evaluated their usage of three transit services: the Bus (BB), the Commuter Rail (RR), and the Express Ferry (FF). The survey revealed the following information:

- 160160 commuters use the Bus.
- 140140 commuters use the Commuter Rail.
- 110110 commuters use the Express Ferry.
- 6060 commuters use both the Bus and the Commuter Rail.
- 4545 commuters use both the Commuter Rail and the Express Ferry.
- 5050 commuters use both the Bus and the Express Ferry.
- 2525 commuters use all three transit services.

How many of the surveyed commuters use none of these three transit services?

Show answer & explanation

Answer: 20

Answer

20 commuters use none of the three transit services.
Using the inclusion-exclusion principle for three overlapping sets, the total number of commuters using at least one of the transit services is 160+140+110(60+45+50)+25=280160 + 140 + 110 - (60 + 45 + 50) + 25 = 280. Subtracting this value from the total surveyed group of 300300 commuters gives 300280=20300 - 280 = 20 commuters who use none of the three services.

Step-by-Step Solution

1
Apply the Principle of Inclusion-Exclusion for three sets to find the total number of commuters who use at least one transit service, BRF|B \cup R \cup F|.
BRF=B+R+F(BR+RF+FB)+BRF|B \cup R \cup F| = |B| + |R| + |F| - (|B \cap R| + |R \cap F| + |F \cap B|) + |B \cap R \cap F|
Simply adding set sizes double-counts elements in pairwise intersections and triple-counts elements in all three sets.
2
Substitute the given numerical values into the inclusion-exclusion formula.
BRF=160+140+110(60+45+50)+25=410155+25=280|B \cup R \cup F| = 160 + 140 + 110 - (60 + 45 + 50) + 25 = 410 - 155 + 25 = 280
Combining the sums and differences yields the exact count of commuters using at least one mode of transit.
3
Subtract the number of commuters using at least one service from the total surveyed population to find those using none.
None=300280=20\text{None} = 300 - 280 = 20
The universe of surveyed commuters consists of those using at least one service plus those using none.

Key Concept

Principle of Inclusion-Exclusion for Three Sets
Estimated Time:1m 30s
Question 17Question

An environmental auditing agency surveyed 250250 manufacturing plants regarding their compliance with three environmental standards: Air Quality (AA), Water Discharge (WW), and Waste Management (MM). The survey yielded the following data:

- 130130 plants meet Air Quality standards (AA).
- 140140 plants meet Water Discharge standards (WW).
- 120120 plants meet Waste Management standards (MM).
- 4040 plants meet all three standards.
- 2020 plants meet none of the three standards.
- The number of plants meeting both Air Quality and Water Discharge standards is equal to the number of plants meeting both Water Discharge and Waste Management standards.
- The number of plants meeting both Air Quality and Waste Management standards is 1010 fewer than the number meeting both Air Quality and Water Discharge standards.

How many of the surveyed plants meet exactly one of the three environmental standards?

Show answer & explanation

Answer: 110

Answer

110
By setting up the 3-set inclusion-exclusion equation, the unknown pairwise intersections are found to be 70, 70, and 60. Subtracting the 40 plants that meet all three standards gives the exclusive double-overlap regions (30, 30, and 20). Subtracting these along with the central intersection from each single set yields 40 plants meeting only Air Quality, 40 meeting only Water Discharge, and 30 meeting only Waste Management, totaling 110 plants.

Step-by-Step Solution

1
Find total number of plants meeting at least one standard
|A ∪ W ∪ M| = 250 - 20 = 230
Subtracting plants that meet no standards from the total surveyed gives the union of all three sets.
2
Set up algebraic expressions for pairwise intersections
|A ∩ M| = k, |A ∩ W| = k + 10, |W ∩ M| = k + 10
Define the smallest pairwise intersection as k and express the other two based on the given relationships.
3
Apply the Principle of Inclusion-Exclusion (PIE) for three sets to solve for k
230 = 130 + 140 + 120 - (k + 10 + k + 10 + k) + 40 => k = 60
Substitute set sizes and the triple intersection into the 3-set inclusion-exclusion formula.
4
Calculate the number of plants in each exclusive region
Only (A ∩ W) = 30, Only (W ∩ M) = 30, Only (A ∩ M) = 20
Subtract the triple intersection (40) from each pairwise intersection.
5
Calculate plants meeting exactly one standard and sum them
Only A = 40, Only W = 40, Only M = 30; Total = 40 + 40 + 30 = 110
Subtract all double-overlap and triple-overlap regions from each individual set total.

Key Concept

Three-Set Principle of Inclusion-Exclusion and Venn Diagram Region Decomposition
Question 18Question

A university surveyed a cohort of 150150 freshmen regarding their membership in three student organizations: the Art Club (AA), the Music Society (MM), and the Theater Guild (TT). The survey revealed the following data:

- 6868 students belong to the Art Club.
- 6262 students belong to the Music Society.
- 5454 students belong to the Theater Guild.
- 2222 students belong to both the Art Club and the Music Society.
- 1818 students belong to both the Music Society and the Theater Guild.
- 1515 students belong to both the Art Club and the Theater Guild.
- 88 students belong to all three organizations.

How many of the surveyed students belong to exactly one of these three organizations?

Show answer & explanation

Answer: 98

Answer

98 students belong to exactly one of the three organizations.
To find the number of students belonging to exactly one organization, analyze the regions of a 3-set Venn diagram starting from the innermost region (all three clubs = 88). Subtracting 88 from each pairwise intersection gives the students in exactly two clubs: Art & Music only (1414), Music & Theater only (1010), and Art & Theater only (77). Next, subtract the overlapping regions from each club total: Art only is 68(14+7+8)=3968 - (14 + 7 + 8) = 39; Music only is 62(14+10+8)=3062 - (14 + 10 + 8) = 30; Theater only is 54(7+10+8)=2954 - (7 + 10 + 8) = 29. Summing these single-club regions gives 39+30+29=9839 + 30 + 29 = 98.

Step-by-Step Solution

1
Find the number of students belonging strictly to each pair of organizations (two-set intersections only).
Art and Music only = 1414; Music and Theater only = 1010; Art and Theater only = 77.
The given pairwise totals include students who belong to all three organizations (88), so subtracting 88 isolates those in exactly two groups.
2
Determine the number of students belonging to each individual organization exclusively.
Art only = 3939; Music only = 3030; Theater only = 2929.
Subtract all shared membership regions (both two-group only and three-group) from each total organization membership.
3
Add the counts of students belonging to exactly one group.
39+30+29=9839 + 30 + 29 = 98.
The question requests the sum of all students in the non-overlapping single-set regions.

Key Concept

Three-Set Venn Diagram Region Partitioning
Question 19Question

At a technology conference attended by 8080 software engineers, 5050 engineers write code in Python, 4040 write code in Java, and 1515 write code in neither Python nor Java. How many of the software engineers write code in both Python and Java?

Show answer & explanation

Answer: 25

Answer

The number of software engineers who write code in both Python and Java is 25.
Out of 8080 engineers, 1515 write neither language, which means 8015=6580 - 15 = 65 engineers write Python, Java, or both. Using the inclusion-exclusion principle, PJ=P+JPJ|P \cup J| = |P| + |J| - |P \cap J|, substituting the values gives 65=50+40PJ65 = 50 + 40 - |P \cap J|. Solving for the overlap gives PJ=9065=25|P \cap J| = 90 - 65 = 25. Thus, 25 engineers write code in both languages.

Step-by-Step Solution

1
Find the number of engineers who write code in at least one of the two languages (Python or Java).
PJ=8015=65|P \cup J| = 80 - 15 = 65
Subtracting the engineers who write neither language from the total gives the union of the two sets.
2
Apply the Principle of Inclusion-Exclusion formula for two sets.
PJ=P+JPJ|P \cup J| = |P| + |J| - |P \cap J|
The total number of engineers in the union is equal to the sum of the individual sets minus their intersection.
3
Substitute the known values into the equation to solve for the intersection PJ|P \cap J|.
65=50+40PJ    65=90PJ    PJ=2565 = 50 + 40 - |P \cap J| \implies 65 = 90 - |P \cap J| \implies |P \cap J| = 25
Solving the linear equation yields the number of engineers writing both languages.

Key Concept

Principle of Inclusion-Exclusion for Two Sets
Estimated Time:1m 0s
Question 20Question

An environmental protection agency audited 240240 coastal wetland sites to evaluate contamination by three specific pollutants: Microplastics (MM), Heavy metals (HH), and Agricultural runoff (AA). The audit revealed the following findings:

- Exactly 3030 of the audited wetlands showed no contamination from any of the three pollutants.
- The total number of wetlands containing Microplastics, Heavy metals, and Agricultural runoff were 130130, 110110, and 100100, respectively.
- The number of wetlands containing Microplastics and Heavy metals but NOT Agricultural runoff was 2525.
- The number of wetlands containing Heavy metals and Agricultural runoff but NOT Microplastics was 3535.
- The number of wetlands containing Microplastics and Agricultural runoff but NOT Heavy metals was 2020.

Based on the audit data, which of the following statements must be true? Select all such statements.

Select all that apply

Show answer & explanation

Answer: Exactly 2525 wetlands contain all three pollutants.; The number of wetlands containing only Microplastics is 6060.; The ratio of wetlands containing only Heavy metals to wetlands containing only Agricultural runoff is 55 to 44.

Answer

The statements confirming that exactly 25 wetlands contain all three pollutants, that 60 wetlands contain only Microplastics, and that the ratio of wetlands containing only Heavy metals to only Agricultural runoff is 5 to 4 are all correct.
The correct options are those stating that 25 wetlands contain all three pollutants, that 60 wetlands contain only Microplastics, and that the ratio of Heavy metals only to Agricultural runoff only is 5 to 4. Each of these follows directly from setting up the inclusion-exclusion equation for three sets and determining all eight mutually exclusive regions of the Venn diagram.

Step-by-Step Solution

1
Determine the number of wetlands containing at least one pollutant.
Total in union MHA=24030=210|M \cup H \cup A| = 240 - 30 = 210.
The total audited set is 240, and 30 sites have no pollutants.
2
Apply the Principle of Inclusion-Exclusion for three sets.
MHA=M+H+A(MH+HA+AM)+MHA|M \cup H \cup A| = |M| + |H| + |A| - (|M \cap H| + |H \cap A| + |A \cap M|) + |M \cap H \cap A|, which simplifies to 210=130+110+100S2+x    S2x=130210 = 130 + 110 + 100 - S_2 + x \implies S_2 - x = 130, where S2S_2 is the sum of pairwise intersections and xx is the triple intersection.
This relates the total union to the individual set sizes and intersection regions.
3
Express S2S_2 in terms of the given 'exactly two' regions and solve for xx.
The number of wetlands with exactly two pollutants is 25+35+20=8025 + 35 + 20 = 80. Since S23x=80S_2 - 3x = 80, substituting S2=130+xS_2 = 130 + x gives (130+x)3x=80    2x=50    x=25(130 + x) - 3x = 80 \implies 2x = 50 \implies x = 25.
The sum of pairwise overlaps counts the triple intersection three times, so subtracting 3x3x yields the 'exactly two' region.
4
Calculate single-category region sizes.
Microplastics only = 130(25+20+25)=60130 - (25 + 20 + 25) = 60; Heavy metals only = 110(25+35+25)=25110 - (25 + 35 + 25) = 25; Agricultural runoff only = 100(20+35+25)=20100 - (20 + 35 + 25) = 20. Total single-category = 60+25+20=10560 + 25 + 20 = 105.
Subtracting all dual and triple overlap counts from each set total isolates the exclusive membership.
5
Verify each choice statement against the calculated region counts.
Triple intersection is 25 (True). Microplastics only is 60 (True). Exactly one pollutant total is 105, not 85 (False). Percentage of total sample with at least two pollutants is 105/240=43.75%105 / 240 = 43.75\%, not 50%50\% (False). Ratio of Heavy metals only to Agricultural runoff only is 25:20=5:425:20 = 5:4 (True).
Validates exact values against option assertions.

Key Concept

Three-set Principle of Inclusion-Exclusion and Venn diagram region decomposition
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