Question

Difficulty: Very hardSets, Set Operations, and Venn Diagrams

In a survey of 120120 town residents regarding three newspapers—*The Herald* (HH), *The Express* (EE), and *The Nation* (NN)—it was found that 5252 read *The Herald*, 4545 read *The Express*, and 6060 read *The Nation*. Furthermore, 1515 read both *The Herald* and *The Express*, 2222 read both *The Express* and *The Nation*, and 1818 read both *The Herald* and *The Nation*. If the number of residents who read none of these three newspapers is twice the number of those who read all three newspapers, how many residents read exactly two of the newspapers?

  1. A
    3131
  2. 3737Answer
  3. C
    4343
  4. D
    5555

Answer

The number of residents who read exactly two newspapers is 3737.
Using the principle of inclusion-exclusion, n(HEN)=52+45+60(15+22+18)+x=102+xn(H \cup E \cup N) = 52 + 45 + 60 - (15 + 22 + 18) + x = 102 + x, where xx is the number of residents reading all three newspapers. Since the number of residents reading none is 2x2x, the total universe equation is 102+x+2x=120102 + x + 2x = 120, giving 3x=183x = 18 and x=6x = 6. The number of residents reading exactly two newspapers is (156)+(226)+(186)=9+16+12=37(15 - 6) + (22 - 6) + (18 - 6) = 9 + 16 + 12 = 37.

Step-by-Step Solution

1
Formulate the principle of inclusion-exclusion for three sets.
n(HEN)=n(H)+n(E)+n(N)n(HE)n(EN)n(HN)+n(HEN)n(H \cup E \cup N) = n(H) + n(E) + n(N) - n(H \cap E) - n(E \cap N) - n(H \cap N) + n(H \cap E \cap N)
To express the total number of residents reading at least one newspaper in terms of the unknown number of residents who read all three.
2
Substitute the given values and set up the equation for the universal set.
n(HEN)=52+45+60152218+x=102+xn(H \cup E \cup N) = 52 + 45 + 60 - 15 - 22 - 18 + x = 102 + x. Let n(HEN)=2xn(H \cup E \cup N)' = 2x. Then 102+x+2x=120    102+3x=120    3x=18    x=6102 + x + 2x = 120 \implies 102 + 3x = 120 \implies 3x = 18 \implies x = 6.
The sum of elements in the union and its complement must equal the universal set size of 120120.
3
Calculate the number of residents reading exactly two newspapers.
(n(HE)x)+(n(EN)x)+(n(HN)x)=(156)+(226)+(186)=9+16+12=37 (n(H \cap E) - x) + (n(E \cap N) - x) + (n(H \cap N) - x) = (15 - 6) + (22 - 6) + (18 - 6) = 9 + 16 + 12 = 37
Subtracting the triple intersection xx from each pairwise intersection isolates the regions corresponding to exactly two newspapers.

Key Concept

Three-set inclusion-exclusion principle and Venn diagram cardinal region decomposition
Estimated Time:1m 30s
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