PrepTest 141, Section 4, 25. Roberta is irritable only…

How will the right answer fit in terms of support and conclusion?

Only the right answer will accurately describe both the support and conclusion, and thus also a flaw in the argument.

Highlight the main conclusion in the passage, if there is one:

she is almost certainly irritable.

[SUPPORT], and [SUPPORT]. Since [SUPPORT], and [SUPPORT], [CONCLUSION].

The first two pieces of support both include “only”, which makes them necessary conditions. If you combine necessary conditions with facts the right way, you can guarantee a conclusion. The prompt says the argument is flawed, so this author must not be combining the conditions and the facts the right way.

Map the wording of the answers to the wording of the passage:

(A) infers…that tiredness causes yawning.

This is pretty tricky, so I get why you might like this one. But Roberta hasn’t just been yawning, she also “lost her keys”, and we know that means she must be tired. So it doesn’t matter if “tiredness causes yawning”, because we know she’s tired even without the bit about her yawning.

(B) assumes the conclusion…

Where does this assume she’s irritable? It definitely doesn’t. This could be tough to recognize even if it’s correct, so it’s a good thing there’s never been an LSAT argument whose flaw was assuming its own conclusion.

(C) generalizes…

Stop. No it doesn’t. The whole thing is specific to Roberta.

(D) takes a necessary condition for Roberta’s losing things to be a sufficient condition

I like the first part about the necessary condition, since that maps to “loses things only when she is tired”. Being tired is a necessary condition for her to lose things. But the next part about the sufficient condition is wrong, since it never says anything like “being tired is enough to guarantee she’s gonna lose something”.

(E) take a necessary condition for Roberta’s being irritable to be a sufficient condition

The necessary condition part is right, since it maps to “Roberta is irritable only when she is tired”. Being tired is a necessary condition for her to be irritable. And the sufficient condition part is correct too, since the argument says the fact that she “just lost her keys” is enough to guarantee that “she is almost certainly irritable”.

(E) is the correct answer.

Common pattern/s in this question: Most people I work with get pretty good at recognizing when an argument makes a conclusion based on a combination of a necessary condition and facts. Maybe because test perp companies overemphasize the hell out of conditional reasoning. On hard ones like this, you can see really carefully checking the wording of the answers is the key ingredient in avoiding a mistake.