Logical Reasoning · Necessary Assumption

PrepTest 152 · Section 2 · Question 12

Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.

Step 1 · Pre-Question Guidance · read this first

Question type

Necessary Assumption

The one idea it tests

When a rule kicks in for things that pass a bar, showing something is 'as good as' another thing only helps if that other thing passes the bar.

The basic logic

The rule: if a thing hits a certain level, then some action follows. The evidence: thing X equals thing Y. The conclusion: the action follows for X. The missing plank: Y actually reaches that level. Pull the Plank: if Y does not reach it, being equal to Y proves nothing.

Tiny words that decide it

“if”
“significantly”
“equal”
“thus”

Playground logic

Aunt Rosa's rule says she packs any snack that truly keeps Tommy full until recess. A taste test shows apple slices fill him up just as much as crackers. The secret sauce: the crackers themselves actually keep him full until recess.

Step 2 · The kid story

The Snack Rule

Aunt Rosa has a rule for Tommy's lunchbox. 'If a snack really keeps you full until recess, I'll pack it.' Tommy wants apple slices, because he can hide candy under them. So he runs a test. Monday he eats crackers. Tuesday he eats apple slices. Both days his tummy rumbles at exactly the same time. 'See?' Tommy says. 'Apples work just as well as crackers! So you have to pack apples!' Lulu taps her clipboard. 'Hang on. Do the crackers keep you full until recess?' Tommy thinks. 'Well... both days I was starving by second period.' Lulu shrugs. 'Then apples being as good as crackers means apples are also not good enough.' Tommy's plan only works if crackers actually pass Aunt Rosa's test.

Step 3 · Try the model question

Aunt Rosa packs any snack that really keeps Tommy full until recess. Tommy's test showed apple slices keep him just as full as crackers do. So Aunt Rosa should pack apple slices.

Tommy's argument depends on which assumption?

Step 4 · The courtroom tale

The Case of the Twin Umbrellas

In a pretend courtroom, a store must refund any umbrella that fails to keep a customer dry. A shopper's lawyer argues, 'Lab tests show the Model B umbrella performs exactly like the Model A umbrella. So Model B must be refunded too.' Mr. Whitfield raises an eyebrow. 'Your Honor, that only works if Model A failed to keep people dry. Did it?' The lawyer flips through papers. 'Um, we never tested that.' The judge sighs. 'If Model A kept people dry, then Model B, being the same, did too. Your argument needs Model A to fail first.' Equal to something is only as good, or bad, as that something.

Step 5 · Work the real question

Open PrepTest 152, Section 2, Question 12 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the rule as if-then and note the bar it sets. Then see what the evidence actually compares: one thing measured against another. Ask whether the second thing has been shown to hit the bar. If not, that is your gap. Predict it, then use Pull the Plank on each choice: negate it and see if the argument falls. Avoid choices that demand more than needed, like being the best or the only.