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PrepTest 73, Logical Reasoning 2, Question 25


Question 25, when a question asks for a reason why the argument is flawed, it's a flaw question, which means we have a bad argument and we need to find out what's bad about it. But of course, that means understanding what the argument says its conclusion and its evidence. Conclusion this argument comes at the end.

Roberta is almost certainly irritable and the evidence that they give us for this, well, the first piece is that Roberta is irritable only when she's tired, and she loses things only when she's tired. So two things she only does when she's tired. And then they inform us that she's been yawning all day and she just lost her keys.

This seems like formal logic. It's because it is. We could take each of the first two statements and make formal logic shorthand out of them. Roberta is irritable only when she's tired and she loses things only when she's tired.

She's been yawning all day, which seems to come out of nowhere, but the important thing is she just lost her keys. So the conclusion is that Roberta is irritable, but we don't have enough information to prove that she's irritable. We only have enough information to prove that she is tired. According to that second rule, she only loses things when she's tired.

And said she just lost something, we would know that she is tired. What we don't know is that she's irritable. Essentially, this is a sufficiency and necessity problem. Her being irritable would be sufficient to prove that she's tired but it's not a necessary consequence of being tired. Just cuz she's tired, I don't know that she's irritable.

So that's what we're gonna look for in the answer choices, a sufficiency necessity error. So answer choice A says that the argument in first from a correlation between tiredness and yawning that tiredness causes yawning. But the argument doesn't say that tiredness causes yawning just says that she is yawning and that she lost her keys.

It doesn't try to make a connection between those two things. So answer choice A is not our answer. Answer choice B, assuming the conclusion that it sets out to prove, this is a circular argument which occurs only when the evidence and conclusion in an argument are identical, and they aren't here. Answer choice C says that the argument generalizes on the basis of a single instance.

There is a generalization as argument, but the argument isn't try to prove the generalizations, the generalizations are things that the argument is using to prove something with. So answer choice C is not our answer. Answer choice D is almost our answer. It does say that the argument is making a necessity sufficient flaw, but this is actually something that the argument does right.

The argument doesn't confuse the necessary condition for losing things with a sufficient condition for losing things. The argument says that she loses things only when she's tired. And it gets it right. She just lost something so she must be tired. That means that the correct answer has to be answer choice E, takes a necessary condition from Roberta's being irritable to be a sufficient condition.

And that was the era that we identified, just because she's tired, I don't know she's irritable. Being tired is a necessary condition for her to be irritable. But I don't know that it is a sufficient condition to prove that she's irritable. So answer choice E is our answer.

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