PrepTest 133 · Section 1 · Question 21 (Parallel Reasoning)

Step 1 · Pre-Question Guidance · read this first

Question type

Parallel Reasoning

The one idea it tests

A two-way split (this case gives one result, every other case gives the other) lets you reason from 'not the second result' to 'the first result.'

The basic logic

X-Ray the argument. Premise: in case one you get result P; in every other case you get result Q. Conclusion: if it is not Q, then it is P. Since every case is one or the other, ruling out Q leaves only P. Find the answer with that same split and that same 'not the other one, so this one' conclusion.

Tiny words that decide it

“any other”
“otherwise”
“not”

Playground logic

Ms. Rivera says rainy days mean gym recess and every other day means playground recess. Lulu concludes: if recess is not on the playground, it must be in the gym. The patty splits all days into two bins, so ruling out one bin leaves the other, and that's the whole burger.

Step 2 · The kid story

Two Bins for Recess

Ms. Rivera has a simple recess rule. 'If it rains, we play in the gym. On any other day, we play on the playground.' Tommy comes back from the dentist at lunchtime and finds the playground empty. 'Where is everybody?' Lulu checks her clipboard. 'Recess is not on the playground today. Every day is either a rain day or a not rain day. Rain days are gym days, and not rain days are playground days. So if it is not playground, it has to be gym.' Marcus scratches his head. 'Could they be at the zoo?' Priya sips her juice box. 'Not under this rule. It only has two bins, and the playground bin is empty.' Tommy jogs to the gym. There they are, playing dodgeball.

Step 3 · Try the model question

If it rains, recess is in the gym. On any other day, recess is on the playground. So if recess is not on the playground, recess is in the gym.

Which one of the following has a pattern of reasoning most like Lulu's?

Step 4 · The courtroom tale

The Two-Door Toll Booth

Judge Kim reviews a parking dispute. The town rule: 'Cars with a resident sticker park in Lot A. All other cars park in Lot B.' A guard testifies, 'This car was not in Lot B.' Mr. Whitfield snaps his suspenders. 'Then we know nothing, Your Honor!' The judge shakes her head. 'Counselor, the rule gives only two lots. Every car has a sticker or it does not. If the car is not in Lot B, the rule puts it in Lot A.' Mr. Whitfield frowns. 'Could it be on the roof?' 'Not under this rule,' says the judge. 'Two doors, one closed. Walk through the other.'

Step 5 · Work the real question

Open PrepTest 133, Section 1, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Strip the argument to letters. Note how the rule sorts every case into two results and what the conclusion rules out and rules in. Then x-ray each answer the same way, writing its premise and conclusion as letters. Pick the one whose conclusion moves from 'not the second result' to 'the first result.' Don't be fooled when several answers share the same premise: the conclusion decides it.