LSAT 15 – Section 3 – Question 18

LSAT 15 - Section 3 - Question 18

June 1995

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Question
QuickView		 Type Tags Answer
Choices		 Curve Question
Difficulty		 Psg/Game/S
Difficulty		 Explanation
PT15 S3 Q18
+LR
Sufficient assumption +SA
A
48%
167
B
2%
157
C
32%
162
D
15%
160
E
4%
156
156
165
174
 +Hardest 147.322 +SubsectionMedium
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We know this question is a sufficient assumption question because the question stem asks which of the answer choices “enable the conclusion to be properly drawn?”

Sufficient assumption questions tend to be very formal. We’re looking for a rule that would validate the conclusion, specifically by bridging the premise and conclusion through the rule. Not only are we extrapolating the rule from our argument, but we’re plugging that rule back into the argument to make it valid. Our rule/prephrase will look like: if [premise], then [conclusion].

Our first sentence describes specific kinds of experiments and the observations from those experiments. The experiments were conducted with certain kinds of bacteria. They were placed around lots of nutrients and two things were observed: population grew (which makes sense, more nutrients = growth), and genetic mutation occurred at random. These observations are our premises.

The next sentence is the hypothesis/conclusion: based on these experiments, the author hypothesized that all genetic mutation is random. All? That’s a big jump from mutations in certain bacteria to all genetic mutations. “All genetic mutations” includes mutations that aren’t just bacteria, too. Our rule would look like: “If certain bacteria genetically mutate at random, then all genetic mutations are random.”

Remember, our paraphrase is meant to guide us through the answer choices. The answer choice may not mimic the paraphrase, but it should make the argument valid when we plug it back into the conclusion.

Correct Answer Choice (A) We know for sure that the genetic mutation did happen in our experiment, and with the dichotomy in answer choice A, we either have to accept that all mutations are random, or none are. Since we already have some random genetic mutations in bacteria in our experiments, we have to accept that all genetic mutations across lifeforms are random; accepting the latter wouldn’t make any sense and isn’t possible. In other words, if we plug this back into our argument, the conclusion is valid.

Answer Choice (B) Just because bacteria used in the experiments are common, it doesn’t mean that random genetic mutation occurring in this instance will occur in all other instances as well.

Answer Choice (C) We can’t trigger the sufficient condition in this conditional. With the information in the stimulus, we only can say that certain bacteria go through genetic mutation. This is useless.

Answer Choice (D) If we plug this back into the premises, given what we know about the massive jump the argument makes between the premise and the conclusion, this answer choice does absolutely nothing. It’s additional information that has no positive or negative bearing on the argument.

Answer Choice (E) Knowing that these bacteria are found in nature isn’t enough; we need to know about genetic mutation across life forms, not just bacteria.

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LSAT PrepTest 15 Explanations

Section 1 - Reading Comprehension

Section 2 - Logical Reasoning

Section 3 - Logical Reasoning

Section 4 - Logic Games

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