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5556 questions

Question 1501Question

The passage below is adapted from an article about twentieth-century astronomy.

In 1964, radio astronomers Arno Penzias and Robert Wilson detected a persistent, low-frequency hiss while testing a highly sensitive horn antenna. They initially blamed the static on nesting pigeons and painstakingly cleaned the instrument. <u>Consequently,</u> the mysterious signal remained, eventually leading to the discovery of cosmic microwave background radiation.

Which choice provides the most logical transition between the two sentences?

Show answer & explanation

Answer: Nevertheless,

Answer

The transition 'Nevertheless,' provides the correct logical relationship of contrast between the cleaning of the antenna and the signal's persistence.
The transition 'Nevertheless,' is correct because it logically connects the contrasting ideas: the antenna was cleaned, but the signal still remained.

Step-by-Step Solution

1
Analyze the relationship between the sentence before the transition and the sentence containing the transition.
The first sentence describes cleaning the antenna to remove noise, while the second sentence describes the noise remaining.
Determining the relationship between these two thoughts is the first step in selecting the correct transition.
2
Identify the logical category required.
Because the outcome (the signal remaining) is the opposite of what would be expected after cleaning the antenna, a contrast transition is needed.
Knowing the correct category helps eliminate transitions from incorrect categories (such as addition or cause-effect).
3
Evaluate the options to find the one that fits the contrast category.
The transition 'Nevertheless,' is the only option that expresses contrast.
This confirms the correct transition choice.

Key Concept

Selecting transition words that establish the correct logical relationship between adjacent sentences.
Question 1502Question

Passage Excerpt:
In 1945, engineer Percy Spencer was working on active radar sets when he noticed that a chocolate bar in his pocket had suddenly melted. Intrigued by this phenomenon, Spencer decided to investigate the heating effect of magnetrons. He placed various foods near the radar equipment, including popping corn, which quickly began to pop and scatter across the lab. [X] Realizing that microwave energy could cook food rapidly, Spencer and his employer, Raytheon, filed a patent for the first commercial microwave oven.

The writer wants to add a sentence at point [X] to emphasize that the food Spencer tested became increasingly complex as his experiments progressed. Given that all the choices are true, which choice best accomplishes this goal?

Show answer & explanation

Answer: Next, he graduated from simple kernels to a raw egg in its shell, observing how the waves cooked the yolk and whites from the inside out until the shell exploded.

Answer

The option that describes graduating from simple kernels to a raw egg in its shell is correct because it directly addresses the rhetorical goal of showing a progression in complexity by moving from simple popcorn kernels to a raw egg, while maintaining proper grammar and past-tense consistency with the rest of the passage.
The option that describes graduating from simple kernels to a raw egg in its shell is correct because it directly addresses the rhetorical goal of showing a progression in complexity by moving from simple popcorn kernels to a raw egg, while maintaining proper grammar and past-tense consistency with the rest of the passage.

Step-by-Step Solution

1
Analyze the rhetorical goal specified in the prompt, which is to emphasize that the food Spencer tested became increasingly complex as his experiments progressed.
Identify that the correct sentence must explicitly show a transition from a simple food item to a more complex food item.
This establishes the criteria needed to evaluate each of the answer choices.
2
Evaluate the choices to see which ones are grammatically correct and consistent with the passage's tense.
Eliminate the choice containing a comma splice ('He then tested a raw egg...').
ACT English requires all sentences to be grammatically correct, even if they appear to meet the rhetorical goal.
3
Evaluate the remaining grammatically correct choices against the specific rhetorical goal.
Select the choice starting with 'Next, he graduated...' because it compares popcorn kernels to a raw egg with distinct parts (yolk and whites), showing a clear progression in complexity.
This choice is the only one that successfully meets the rhetorical requirement of indicating increasingly complex food items.

Key Concept

Meeting Specific Rhetorical Goals
Question 1503Question

[1] In 1998, an anonymous collector purchased a damaged medieval prayer book at auction for two million dollars, initiating one of the most complex projects in modern conservation. [2] The book was a palimpsest—a manuscript whose original text had been scraped away to reuse the parchment. [3] Beneath the thirteenth-century prayers lay the only surviving copies of several treatises by the ancient Greek mathematician Archimedes. [4] Conservationists immediately began a decade-long effort to separate the bound pages and read the erased writing. [5] Archimedes is widely regarded as one of the leading scientists of classical antiquity, alongside figures like Euclid and Eratosthenes. [6] Ultimately, by utilizing multispectral imaging and X-ray fluorescence, the team successfully recovered the hidden mathematical texts.

The writer is considering deleting Sentence 5 from the passage. Should this sentence be kept or deleted?

Show answer & explanation

Answer: Deleted, because it interrupts the paragraph's focus on the conservation and recovery of the Archimedes Palimpsest with tangential biographical information.

Answer

Deleted, because it interrupts the paragraph's focus on the conservation and recovery of the Archimedes Palimpsest with tangential biographical information.
The correct option is the one stating that the sentence should be deleted because it interrupts the paragraph's focus on the conservation and recovery of the Archimedes Palimpsest. The paragraph is strictly about the physical work and technical methods used to salvage the manuscript; introducing details about Euclid, Eratosthenes, and Archimedes's general standing in classical antiquity distracts from this unified theme.

Step-by-Step Solution

1
Determine the primary focus of the paragraph.
The paragraph describes the discovery, physical nature, and technical recovery of the Archimedes Palimpsest manuscript.
Understanding the paragraph's main theme is necessary to evaluate the relevance of the sentence in question.
2
Analyze the content of the sentence proposed for deletion.
Sentence 5 offers broad biographical context, comparing Archimedes to other ancient scientists.
This determines whether the sentence contributes to the main narrative or introduces off-topic material.
3
Evaluate the options to match the correct reasoning.
The sentence should be deleted because it breaks the narrative flow regarding manuscript conservation by shifting focus to general history.
A deletion choice must be paired with an accurate justification of why the sentence is not relevant to the specific context.

Key Concept

Evaluating relevance to maintain paragraph unity and cohesion
Question 1504Question

Botanical illustrators play a vital role in scientific communication by translating complex plant structures into clear, detailed drawings. Unlike photographs, which can capture distracting background elements or temporary damage to a leaf, a botanical illustration highlights only the essential diagnostic features of a specimen. Historically, explorers and scientists relied entirely on these artists to document new species. To produce these works, illustrators must possess both artistic skill and a deep understanding of plant anatomy. Even today, modern herbaria continue to commission hand-drawn illustrations because they are way better at showing details that a camera might obscure.

Which option best maintains the formal, academic tone of the passage?

Show answer & explanation

Answer: more effectively clarify

Answer

The option that reads 'more effectively clarify' is the correct choice.
The option containing the phrase 'more effectively clarify' is correct because it uses precise, formal language that matches the academic tone of the passage while remaining concise and grammatically correct.

Step-by-Step Solution

1
Analyze the context and tone of the passage.
The passage discusses the scientific and historical role of botanical illustrators in a formal, objective, and academic tone.
Establishing the tone of the surrounding text is necessary to determine which option is stylistically appropriate.
2
Evaluate the underlined phrase and the choices for tone, conciseness, and clarity.
The original phrase is too colloquial. The option containing 'possess a superior capacity' is overly wordy. The option with 'are more successful and better' is redundant. The option with 'more effectively clarify' is concise and formal.
This helps eliminate options that introduce slang, wordiness, or redundancy.
3
Verify grammatical correctness of the selected option.
Plugging 'more effectively clarify' into the sentence makes it: 'illustrations because they more effectively clarify details', which is grammatically correct and stylistically appropriate.
Ensuring the selected choice integrates seamlessly into the sentence structure without errors.

Key Concept

Stylistic Appropriateness
Estimated Time:45s
Question 1505Question

What is the value of the expression 3×(47)2+24234-3 \times (4 - 7)^2 + \frac{24}{2^3 - 4}?

Show answer & explanation

Answer: 21-21

Answer

The correct answer is 21-21.
Following the order of operations, we first compute the term inside the parentheses: 47=34 - 7 = -3. Next, we evaluate the exponents: (3)2=9(-3)^2 = 9 and 23=82^3 = 8. This simplifies the expression to 3×9+2484-3 \times 9 + \frac{24}{8 - 4}. Simplifying the denominator gives 84=48 - 4 = 4. We then perform the multiplication and division: 3×9=27-3 \times 9 = -27 and 244=6\frac{24}{4} = 6. Adding these results together yields 27+6=21-27 + 6 = -21.

Step-by-Step Solution

1
Evaluate the subtraction inside the parentheses.
47=34 - 7 = -3
According to the order of operations, grouping symbols such as parentheses must be evaluated first.
2
Evaluate the exponential terms in the expression.
(3)2=9(-3)^2 = 9 and 23=82^3 = 8
Exponents must be evaluated after parentheses and before multiplication, division, addition, or subtraction.
3
Simplify the denominator of the fraction.
84=48 - 4 = 4
The denominator must be fully simplified before dividing the numerator by it.
4
Perform multiplication and division from left to right.
3×9=27-3 \times 9 = -27 and 244=6\frac{24}{4} = 6
Multiplication and division have higher priority than addition and subtraction.
5
Perform the final addition.
27+6=21-27 + 6 = -21
Addition and subtraction are performed last.

Key Concept

Order of Operations (PEMDAS)
Question 1506Question

The following paragraph contains numbered sentences.

[1] In 1967, postgraduate student Jocelyn Bell Burnell was analyzing data from a new radio telescope she had helped build. [2] She noticed a repeating, anomalous signal on the chart recorder papers, which she referred to as a "bit of scruff." [3] This signal was incredibly precise, pulsing regularly at intervals of 1.33 seconds. [4] Her advisor, Antony Hewish, initially dismissed the rapid pulses as man-made radio interference. [5] Eventually, the discovery was recognized as a breakthrough in astrophysics, earning Hewish the Nobel Prize in 1974.

The writer wants to add the following sentence to the paragraph:

"Because of this regularity, she initially nicknamed the signal 'LGM-1' for 'Little Green Men,' half-jokingly implying an extraterrestrial origin."

To make the paragraph most logical, this sentence should be placed after which sentence?

Show answer & explanation

Answer: Sentence 3.

Answer

The sentence should be placed after Sentence 3.
Placing the sentence after Sentence 3 is correct because 'this regularity' directly links back to the description in Sentence 3 that the signal was 'pulsing regularly at intervals of 1.33 seconds.' This placement provides immediate context for the nickname 'LGM-1' before transitioning to her advisor's reaction in Sentence 4.

Step-by-Step Solution

1
Analyze the target sentence for transition words and cohesive devices.
The target sentence begins with 'Because of this regularity,' indicating it must follow a sentence that describes a repeating or consistent pattern.
Cohesive markers like 'this regularity' require a clear, preceding noun phrase that establishes the pattern.
2
Scan the paragraph to find where the regular nature of the signal is first established.
Sentence 3 describes the signal as 'pulsing regularly at intervals of 1.33 seconds,' which is the first and only mention of regularity prior to Sentence 4.
This establishes Sentence 3 as the direct antecedent for 'this regularity' in the target sentence.
3
Evaluate the narrative flow with the target sentence inserted after Sentence 3.
Inserting the sentence here creates a logical sequence: the discovery of the regular signal (Sentence 3) leads to the nickname 'LGM-1' (target sentence), which then explains why her advisor was skeptical of its natural origin (Sentence 4).
This maintains the chronological progression of the discovery, reaction, and ultimate historical outcome.

Key Concept

Identifying transitions and reference clues to determine logical sentence placement
Estimated Time:1m 0s
Question 1507Question

Although the chemist had analyzed the liquid compound in the beaker multiple times, she resolved to test the substance once again in order to re-examine the chemical solution a second time.

Which of the following choices best replaces the underlined portion to make the sentence clear and concise?

Show answer & explanation

Answer: once more

Answer

The correct option is 'once more' because it makes the sentence concise and removes the repetitive phrases without introducing any grammatical errors.
The correct option, which replaces the underlined text with 'once more', successfully removes the unnecessary repetition of 'once again', 're-examine', and 'a second time' while keeping the sentence grammatically sound.

Step-by-Step Solution

1
Identify the redundant elements in the underlined portion.
The phrase contains 'once again', 're-examine', 'chemical solution' (which is synonym redundancy for the already mentioned 'substance'), and 'a second time'. These all repeat the idea of testing the compound again.
Redundancy makes a sentence wordy and violates the ACT English standards of conciseness.
2
Evaluate the options for conciseness and grammatical correctness.
The option containing 'once more' removes all the repetitive elements ('once again', 're-examine', 'a second time') and leaves a grammatically clean independent clause. The other options either maintain the redundancy or introduce grammatical errors like a comma splice or a fragment.
To be correct on the ACT, an option must be both grammatically correct and free of unnecessary repetition.

Key Concept

Eliminating redundancy and avoiding grammatical errors such as comma splices and dependent clause fragments.
Question 1508Question

For a positive integer nn, the greatest common factor of nn and 120120 is 1515, and the least common multiple of nn and 120120 is 360360. What is the value of nn?

Show answer & explanation

Answer: 45

Answer

45
The correct answer is 45. The relationship between two positive integers aa and bb and their GCF and LCM is a×b=GCF(a,b)×LCM(a,b)a \times b = \text{GCF}(a, b) \times \text{LCM}(a, b). Substituting a=na = n, b=120b = 120, GCF=15\text{GCF} = 15, and LCM=360\text{LCM} = 360 gives n×120=15×360n \times 120 = 15 \times 360. Dividing both sides of the equation by 120 yields n=45n = 45.

Step-by-Step Solution

1
Identify the given values and the mathematical relationship between two positive integers, their greatest common factor (GCF), and their least common multiple (LCM).
Given a=na = n, b=120b = 120, GCF(n,120)=15\text{GCF}(n, 120) = 15, and LCM(n,120)=360\text{LCM}(n, 120) = 360. The relationship is a×b=GCF(a,b)×LCM(a,b)a \times b = \text{GCF}(a, b) \times \text{LCM}(a, b).
This formula provides a direct algebraic path to solve for the unknown integer when the other parameters are known.
2
Substitute the given values into the relationship formula.
n×120=15×360n \times 120 = 15 \times 360
To set up an equation containing only the unknown variable nn.
3
Isolate and solve for nn.
n=15×360120=15×3=45n = \frac{15 \times 360}{120} = 15 \times 3 = 45
To find the specific integer value of nn.

Key Concept

The product of two positive integers is equal to the product of their greatest common factor (GCF) and their least common multiple (LCM).

Alternative Method

Another way to solve this is to write the prime factorizations. Since 120=23×3×5120 = 2^3 \times 3 \times 5, and their GCF is 15=3×515 = 3 \times 5, nn must contain 3×53 \times 5 but cannot contain any factor of 2. Since their LCM is 360=23×32×5360 = 2^3 \times 3^2 \times 5, nn must contain 323^2 because 120 only has 313^1. Thus, the prime factorization of nn is 32×5=453^2 \times 5 = 45.
Estimated Time:1m 0s
Question 1509Question

A writer is editing an essay about the history of the crossword puzzle and has drafted the following paragraph:

Arthur Wynne, an editor at the New York World, was tasked in 1913 with creating a new game for the newspaper’s Sunday fun section. Remembering a game called "magic squares" from his childhood in England, Wynne designed a diamond-shaped grid with blank spaces and clues to help readers fill in the letters. He called his creation a "Word-Cross." The game was an instant success, prompting readers to write in for more puzzles, which eventually led to the standardized crossword puzzles we solve today.

The writer is considering adding the following sentence to the paragraph (placed after the sentence "He called his creation a 'Word-Cross.'"):

"Wynne's father, a local metalworker, had also taught him how to play chess and checkers during their family travels."

Should the writer make this addition?

Show answer & explanation

Answer: No, because it interrupts the narrative focus on the creation of the crossword puzzle with unrelated details about Wynne's father and other games.

Answer

No, because it interrupts the narrative focus on the creation of the crossword puzzle with unrelated details about Wynne's father and other games.
The correct option is the one stating that the sentence should not be added because it interrupts the narrative focus with unrelated details. The paragraph focuses on how Arthur Wynne created the crossword puzzle in 1913 and how it became popular. Mentioning his father's profession and other games like chess and checkers is tangential and distracts the reader from the main narrative.

Step-by-Step Solution

1
Analyze the main focus of the existing paragraph.
The paragraph focuses on the origin of the crossword puzzle, detailing how Arthur Wynne designed it in 1913 and its subsequent success.
To evaluate relevance, you must first establish the central topic of the passage.
2
Evaluate the content of the proposed sentence.
The proposed sentence introduces information about Wynne's father's profession (metalworker) and other games he learned (chess and checkers).
This step determines whether the new information aligns with or departs from the established focus.
3
Compare the proposed sentence with the paragraph's focus to make a decision and identify the correct justification.
The sentence should not be added because it introduces off-topic details about Wynne's family and unrelated games, which interrupts the paragraph's focus on the crossword puzzle. The correct choice must reflect this decision and logical reasoning.
This allows you to match the correct reasoning with the choices without relying on superficial keyword matches.

Key Concept

Evaluating Relevance for Adding or Deleting Content
Question 1510Question

What is the value of the expression below?

2412+(2)3(1)5×32(2)3÷(131) \frac{-2^4 - |-12 + (-2)^3|}{(-1)^5 \times 3^2 - (-2)^3} \div \left( \frac{1}{3} - 1 \right)
Show answer & explanation

Answer: 54-54

Answer

54-54
Evaluating the numerator gives 36-36, and the denominator simplifies to 1-1, resulting in a fraction of 3636. Dividing 3636 by (1/31)=2/3(1/3 - 1) = -2/3 is performed by multiplying 3636 by the reciprocal 3/2-3/2, which yields the final result of 54-54.

Step-by-Step Solution

1
Evaluate the numerator of the first fraction.
Numerator = 36-36
First, evaluate the exponent inside the absolute value: (2)3=8(-2)^3 = -8. Next, evaluate the expression inside the absolute value: 12+(8)=20-12 + (-8) = -20. Taking the absolute value gives 20=20|-20| = 20. Separately, evaluate the exponent 24=(24)=16-2^4 = -(2^4) = -16 because exponentiation has priority over negation. Finally, subtract the absolute value term from this to get 1620=36-16 - 20 = -36.
2
Evaluate the denominator of the first fraction.
Denominator = 1-1
Evaluate the exponents first: (1)5=1(-1)^5 = -1, 32=93^2 = 9, and (2)3=8(-2)^3 = -8. Then, perform the multiplication: 1×9=9-1 \times 9 = -9. Finally, perform the subtraction: 9(8)=9+8=1-9 - (-8) = -9 + 8 = -1.
3
Simplify the first fraction.
Fraction = 3636
Divide the numerator by the denominator: 361=36\frac{-36}{-1} = 36.
4
Evaluate the expression inside the divisor parentheses.
Divisor = 23-\frac{2}{3}
Perform the fraction subtraction: 131=1333=23\frac{1}{3} - 1 = \frac{1}{3} - \frac{3}{3} = -\frac{2}{3}.
5
Divide the simplified fraction by the divisor.
Final Value = 54-54
To divide by a fraction, multiply by its reciprocal: 36÷(23)=36×(32)=18×(3)=5436 \div \left(-\frac{2}{3}\right) = 36 \times \left(-\frac{3}{2}\right) = 18 \times (-3) = -54.

Key Concept

Order of Operations (PEMDAS) and Number Properties
Question 1511Question

What is the sum of the distinct prime factors of 3030?

Show answer & explanation

Answer: 10

Answer

The sum of the distinct prime factors of 3030 is 1010.
The prime factorization of 3030 is 2×3×52 \times 3 \times 5. The distinct prime factors are 22, 33, and 55, and their sum is 1010.

Step-by-Step Solution

1
Find the prime factorization of 3030.
30=2×3×530 = 2 \times 3 \times 5
To break the composite number 3030 down into its prime components.
2
Identify the distinct prime factors.
The distinct prime factors are 22, 33, and 55.
Only prime numbers that divide 3030 should be included in the sum.
3
Add the distinct prime factors.
2+3+5=102 + 3 + 5 = 10
To calculate the final sum as requested by the question.

Key Concept

Prime factorization is the process of factoring a composite number into a product of prime numbers. The distinct prime factors of a number are the unique prime numbers that divide it.
Question 1512Question

On a standard number line, point AA is located at 7-7 and point BB is located at 55. What is the distance between point AA and point BB?

Show answer & explanation

Answer: 12

Answer

The distance between the two points is 12.
The distance between two points on a number line is the absolute value of their difference. Calculating 75|-7 - 5| yields 12|-12|, which is 1212. Alternatively, subtracting the lesser coordinate from the greater coordinate gives 5(7)=5+7=125 - (-7) = 5 + 7 = 12.

Step-by-Step Solution

1
Identify the coordinates of the two points on the number line.
The coordinates are 7-7 for point AA and 55 for point BB.
These coordinates represent the positions from which we need to find the distance.
2
Apply the absolute value formula for the distance between two points, ab|a - b|.
The expression is 75|-7 - 5|.
Distance on a number line is always non-negative and is defined by the absolute value of the difference between the two coordinates.
3
Perform the subtraction and find the absolute value.
12=12|-12| = 12.
Subtracting 55 from 7-7 gives 12-12, and the absolute value of 12-12 is 1212.

Key Concept

The distance between two points aa and bb on a number line is given by ab|a - b|.
Estimated Time:45s
Question 1513Question

To avoid costly fines, the factory manager ensured that the new manufacturing process was in strict compliance to the environmental regulations regarding waste disposal.

Which of the following options is the most idiomatically correct choice for the underlined portion?

Show answer & explanation

Answer: compliance with

Answer

compliance with
The correct choice replaces the underlined phrase with 'compliance with', which is the standard idiomatic construction in English for expressing agreement or conformity with rules, laws, or standards.

Step-by-Step Solution

1
Identify the underlined phrase and the noun it relies on.
The underlined phrase is 'compliance to', which is built around the noun 'compliance'.
Understanding the head word is necessary to determine the correct idiomatic preposition that must follow it.
2
Recall the correct idiomatic preposition collocation for the noun 'compliance' when referring to following regulations.
The noun 'compliance' idiomatically pairs with the preposition 'with' (e.g., 'in compliance with regulations').
Using any other preposition (like 'to', 'of', or 'for') violates standard English idiomatic usage.

Key Concept

Idiomatic Preposition Collocation
Question 1514Question

In 1911, physicist Ernest Rutherford proposed a nuclear model of the atom based on his famous gold foil experiment. He observed that while most alpha particles passed straight through the foil, a small fraction was deflected at large angles. Due to the reason that the vast majority of the atom's mass is concentrated in a tiny nucleus, the deflective behavior surprised the scientific community. Which choice most clearly and concisely replaces the underlined portion of the sentence?

Show answer & explanation

Answer: Because the vast majority of the atom's mass is concentrated in a tiny nucleus,

Answer

The option starting with 'Because' is the correct choice because it is the most concise phrasing that is grammatically correct and preserves the cause-and-effect relationship of the sentence.
The correct option provides the most direct and concise way to establish the cause-and-effect relationship. It replaces the wordy and redundant introductory phrase with the single conjunction 'Because' while keeping the clause grammatically complete.

Step-by-Step Solution

1
Analyze the underlined portion for conciseness and redundancy.
The original phrase 'Due to the reason that' is wordy and redundant because 'due to' and 'reason' repeat the same concept of causality.
ACT English style requires selecting the most concise phrasing that preserves the intended meaning.
2
Evaluate the grammar of the remaining choices.
The choice using 'Because' is concise and maintains a complete dependent clause. The choice adding 'which is concentrated' removes the main verb of the clause, resulting in a fragment. The choice without a conjunction creates a comma splice by linking two independent clauses together with only a comma.
The correct option must be grammatically sound and avoid fragment or run-on errors.

Key Concept

Eliminating Wordiness
Question 1515Question

An algebra student is simplifying the expression a(bc)d(ef)a(b - c) - d(e - f) by applying the distributive property. Instead of distributing correctly, the student incorrectly writes the expansion as a(bc)dedfa(b - c) - de - df. If a=2a = 2, b=3b = 3, c=5c = 5, d=3d = -3, e=4e = 4, and f=2f = -2, what is the absolute difference between the student's incorrect result and the correct value of the expression?

Show answer & explanation

Answer: 12

Answer

The correct answer is 12.
The correct expression evaluates to 14, and the incorrect expression evaluates to 2. The absolute difference between them is 142=12|14 - 2| = 12.

Step-by-Step Solution

1
Evaluate the correct expression a(bc)d(ef)a(b - c) - d(e - f) using the given values.
14
To find the mathematically correct value of the expression.
2
Evaluate the incorrect expression a(bc)dedfa(b - c) - de - df using the same values.
2
To find the value resulting from the student's distribution error.
3
Find the absolute difference between the correct value and the incorrect value.
12
To determine the error magnitude as requested by the question.

Key Concept

Distributive Property and Order of Operations
Question 1516Question

A student is adding the fractions 512\frac{5}{12} and 718\frac{7}{18}. What is the least common denominator they should use to add these fractions?

Show answer & explanation

Answer: 36

Answer

36
To find the least common denominator of the two fractions, find the least common multiple (LCM) of the denominators, 12 and 18. The prime factorization of 12 is 22×32^2 \times 3, and the prime factorization of 18 is 2×322 \times 3^2. The LCM is found by taking the highest power of each prime factor that appears in either factorization: 22×32=4×9=362^2 \times 3^2 = 4 \times 9 = 36.

Step-by-Step Solution

1
Identify the denominators of the fractions to find their least common denominator.
The denominators are 12 and 18.
The least common denominator of two fractions is equal to the least common multiple (LCM) of their denominators.
2
Find the prime factorization of each denominator.
12=22×312 = 2^2 \times 3 and 18=2×3218 = 2 \times 3^2
Prime factorization helps systematically determine the least common multiple.
3
Calculate the least common multiple by multiplying the highest power of each prime factor present.
22×32=4×9=362^2 \times 3^2 = 4 \times 9 = 36
This yields the smallest positive integer that is divisible by both 12 and 18.

Key Concept

Finding the least common denominator by calculating the least common multiple (LCM) of the denominators.
Estimated Time:45s
Question 1517Question

If xx, yy, and zz are integers such that 5x<y<z5-5 \leq x < y < z \leq 5, what is the minimum possible value of the expression (xy)2z(yx)xyz(x - y)^2 - z(y - x) - |x| \cdot |y - z|?

Show answer & explanation

Answer: -49

Answer

The minimum possible value of the expression is 49-49.
The correct value is 49-49. By setting d1=yx1d_1 = y - x \geq 1 and d2=zy1d_2 = z - y \geq 1, we can rewrite the expression as d12zd1xd2d_1^2 - z d_1 - |x| d_2. Substituting z=x+d1+d2z = x + d_1 + d_2 gives xd1d2(d1+x)-x d_1 - d_2(d_1 + |x|). To minimize this, we choose the smallest possible value for xx, which is 5-5. This gives 5d1d2(d1+5)5d_1 - d_2(d_1 + 5). Since x=5x = -5 and z5z \leq 5, we have d1+d210d_1 + d_2 \leq 10. Substituting d2=10d1d_2 = 10 - d_1 yields 5d1(10d1)(d1+5)=d12505d_1 - (10 - d_1)(d_1 + 5) = d_1^2 - 50. The minimum value of this quadratic expression for integer d11d_1 \geq 1 occurs at d1=1d_1 = 1, giving 1250=491^2 - 50 = -49. This corresponds to x=5,y=4,z=5x = -5, y = -4, z = 5. Evaluating the expression directly confirms this result: (5(4))25(4(5))545=1545=49(-5 - (-4))^2 - 5(-4 - (-5)) - |-5| \cdot |-4 - 5| = 1 - 5 - 45 = -49.

Step-by-Step Solution

1
Define variables for the differences between the ordered integers.
Let d1=yx1d_1 = y - x \geq 1 and d2=zy1d_2 = z - y \geq 1, which implies y=x+d1y = x + d_1 and z=x+d1+d2z = x + d_1 + d_2.
Using differences simplifies the inequality constraints and allows us to express the objective function in terms of positive integers d1d_1 and d2d_2.
2
Substitute the differences into the given expression (xy)2z(yx)xyz(x - y)^2 - z(y - x) - |x| \cdot |y - z|.
The expression becomes d12(x+d1+d2)d1xd2=xd1d2(d1+x)d_1^2 - (x + d_1 + d_2)d_1 - |x|d_2 = -x d_1 - d_2(d_1 + |x|).
This rewrites the expression in terms of the initial variable xx and the positive differences d1d_1 and d2d_2.
3
Analyze how to minimize xd1d2(d1+x)-x d_1 - d_2(d_1 + |x|) given the boundaries.
Setting x=5x = -5 makes the expression 5d1d2(d1+5)5d_1 - d_2(d_1 + 5). Since x=5x = -5 and z5z \leq 5, we have d1+d210d_1 + d_2 \leq 10.
Choosing the minimum value for xx maximizes the positive coefficient of d1d_1 and the term x|x| in the negative product, leading to the smallest possible value.
4
Maximize d2d_2 by setting d2=10d1d_2 = 10 - d_1 and substitute it into the expression.
The expression becomes 5d1(10d1)(d1+5)=d12505d_1 - (10 - d_1)(d_1 + 5) = d_1^2 - 50.
Since d2d_2 has a negative coefficient, maximizing d2d_2 minimizes the overall expression.
5
Minimize d1250d_1^2 - 50 subject to d11d_1 \geq 1.
The minimum occurs at d1=1d_1 = 1, yielding 1250=491^2 - 50 = -49, which corresponds to x=5,y=4,z=5x = -5, y = -4, z = 5.
Since d12d_1^2 is strictly increasing for positive integers, the minimum value of the quadratic is achieved at the smallest boundary value of d1d_1.

Key Concept

Order of Operations and Number Properties
Question 1518Question

The treaty was signed in 1919, but its terms were so harsh that they [underlined]fertilized[/underlined] the very instability they were meant to prevent. This systemic fragility ultimately paved the way for another global conflict, demonstrating how retributive peace agreements can yield catastrophic consequences. Historians continue to study this period to understand the delicate balance between justice and reconciliation.

Which choice most precisely and clearly completes the sentence?

Show answer & explanation

Answer: fostered

Answer

fostered
The word 'fostered' is correct because it precisely means to encourage or promote the development of a state or condition, which fits the context of treaty terms leading to instability. It also maintains the formal, academic tone of the passage.

Step-by-Step Solution

1
Analyze the context of the sentence and the meaning of the underlined word.
The sentence discusses how the harsh terms of a treaty encouraged or promoted the development of instability. The word 'fertilized' is an agricultural term and is imprecise when used to describe political instability.
To identify if a correction is needed.
2
Evaluate the other options to find a word that is precise, clear, and maintains the proper tone.
'Cultivated' is incorrect because it implies positive, purposeful growth. 'Cooked up' is too informal and violates the academic tone. 'Fostered' is precise and fits the register perfectly.
To select the option that best fits the sentence's meaning and register.

Key Concept

Word Precision and Clarity
Question 1519Question

To protect the colony's brood from extreme heat, honeybees perform a behavior known as fanning. By beating their wings rapidly in a coordinated fashion, they can cool down the temperature of the hive and lower it to a safe level. Which of the following choices best replaces the underlined portion?

Show answer & explanation

Answer: they can cool the hive

Answer

The correct choice is the option that states 'they can cool the hive' because it removes the redundant repetition of lowering the temperature while keeping the sentence grammatically complete and structurally sound.
The option stating 'they can cool the hive' is correct because it expresses the intended meaning clearly and concisely without repeating synonyms like 'cool down' and 'lower it,' and it maintains proper sentence structure.

Step-by-Step Solution

1
Identify the redundant expressions in the underlined portion of the sentence.
The phrases 'cool down the temperature' and 'lower it' both describe the process of reducing the heat inside the hive, repeating the same concept unnecessarily.
ACT English requires expressing ideas as concisely as possible without repeating synonyms or concepts.
2
Evaluate the grammar of the remaining concise options to ensure they form a complete sentence.
The option stating 'they can cool the hive' forms a proper independent clause that connects logically to the introductory modifying phrase, whereas the other choices introduce either a comma splice or a sentence fragment.
The correct choice must be both concise and grammatically correct.

Key Concept

Eliminating Redundancy
Question 1520Question

By analyzing the microscopic air bubbles trapped in ancient ice sheets, glaciologists can reconstruct the atmospheric composition of Earth from hundreds of thousands of years ago. These ice cores serve as invaluable archives of historical climate data. Researchers must extract and store these samples under strict temperature controls, making sure they don't mess up the delicate chemical signatures preserved within.

Which choice best maintains the established tone of the passage?

Show answer & explanation

Answer: to prevent the degradation of

Answer

to prevent the degradation of
The correct answer maintains the objective, academic, and scientific tone established throughout the passage. The phrase is concise, formal, and grammatically sound.

Step-by-Step Solution

1
Analyze the context and establish the tone of the passage.
The passage uses words like 'reconstruct,' 'atmospheric composition,' and 'invaluable archives,' establishing a formal, objective, and academic tone.
Identifying the established tone allows for the elimination of options that deviate from this register.
2
Evaluate the underlined phrase and the options for tone consistency.
The phrase 'making sure they don't mess up' is too informal, and 'in order to forestall the catastrophic annihilation of' is too dramatic and pretentious.
This narrows the choices to options that match the academic and scientific register.
3
Evaluate the remaining options for grammatical and idiomatic correctness.
The phrasing 'prevent against' is unidiomatic in English. The verb 'prevent' should act directly on the object ('prevent the degradation of').
Ensuring the selected choice is grammatically and idiomatically correct is necessary to find the right answer.

Key Concept

Tone Consistency
Estimated Time:1m 15s
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