Logical Reasoning · Flaw
PrepTest 157 · Section 2 · Question 9
Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.
Step 1 · Pre-Question Guidance · read this first
- “will ensure”
- “instead”
- “only”
Playground logic
Marcus's patty: the hamster chews its bars after one long lonely week. His top bun: three short lonely stretches mean no chewing ever. The secret sauce he never checked is that the trouble belongs only to the long stretch, which is a Bad Comparison.
Step 2 · The kid story
Peanut and the Three Weekends
The class hamster, Peanut, hates being alone. After the class left him by himself for a whole week of vacation, Ms. Rivera came back to find the cage bars chewed and Peanut sulking in his wheel. Marcus jumped up. "I've solved it! Next time, we leave him alone for three short weekends instead of one long week. Then Peanut will never chew anything again. Guaranteed!" Priya sipped her juice box. "Marcus, how do you know he only chews after long stretches? Maybe he chews after any time alone. Then three weekends could mean three chewing parties." Lulu wrote on her clipboard: Problem seen in plan one. Not checked in plan two. Marcus frowned. "But the weekends are shorter!" "Shorter isn't the same as zero," said Lulu. Peanut, listening, gnawed thoughtfully on a carrot.
Step 3 · Try the model question
Marcus: Peanut chewed his cage bars after being left alone for one long week. So we should leave him alone for three short weekends instead. That plan guarantees he will never chew his bars while we are gone.
Marcus's reasoning is open to which criticism?
Step 4 · The courtroom tale
The Case of the Leaky Boat Plan
Ms. Dunmore, a harbor manager, testified: "Our rowboats leak after sitting in the water all winter. So we will now dock them for three separate months instead. That guarantees no leaks." Mr. Whitfield snapped his suspenders. "Ms. Dunmore, did you check whether boats leak after one month in the water?" "Well, no. But one month is shorter." "Your Honor, the witness found a problem with the long stay and simply assumed the short stays are leak-free. Maybe a month is plenty of time to soak a hull. She compared two plans and pinned the trouble on only one without a single test." The judge nodded. "Shorter is not the same as safe."
Step 5 · Work the real question
Open PrepTest 157, Section 2, Question 9 in your official copy from LSAC (for example in LawHub) and read it there.
Your answer
How to apply it
Find the two setups the argument compares. Note where the evidence shows the bad result: usually in just one of them. Then read the conclusion and see that it promises the other setup is free of that result. Ask whether the argument ever tested the second setup. If not, look for the answer that says the arguer assumed the undesirable outcome goes with only one of the two situations. Skip answers about samples or values that the argument never raises.