Logical Reasoning · Necessary Assumption

PrepTest 110 · Section 2 · Question 13

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

A test on one slice of a group can only challenge a claim about the whole group if that slice is typical.

The basic logic

A broad theory predicts an overall drop. Evidence from one corner shows no drop there. The conclusion says the theory is not fully right. That only works if the corner reflects the whole picture. Pull the plank: if the corner is unusual, the argument falls.

Tiny words that decide it

“total”
“however”
“therefore”
“not entirely”

Playground logic

Grandpa's rule says more homework means fewer kids on the playground overall (bottom bun). Patty: after more homework, the swings had as many kids as before. Top bun: Grandpa's rule is not quite right. Secret sauce: the swings are a fair picture of the whole playground.

Step 2 · The kid story

Grandpa's Playground Rule

Grandpa has a rule: 'When teachers pile on homework, fewer kids show up at the playground.' Then Ms. Rivera doubled the homework. Marcus went to check. He counted the kids on the swings. Twelve, the same as last week! 'Grandpa's rule is busted,' Marcus says. Lulu flips her clipboard. 'Did you count the sandbox? The slide? The basketball court?' 'No, just the swings. The swings are the best part.' Lulu writes slowly. 'Maybe the swings are always full because everyone squeezes in a quick swing before going home. Maybe the basketball court went from twenty kids to two.' Priya agrees. 'Your swing count only matters if the swings show what is happening across the whole playground.' Marcus runs back to count the court.

Step 3 · Try the model question

Grandpa's rule says that more homework means fewer kids at the playground overall. After Ms. Rivera doubled the homework, the swings had about as many kids as before. So Grandpa's rule is not entirely right.

Marcus's argument relies on which assumption?

Step 4 · The courtroom tale

The One Shelf Survey

A bookstore chain claimed in court that raising book prices cuts how many books it sells overall. The opposing lawyer presented evidence: after a price rise, the cookbook shelf sold just as many as before. 'So the chain's claim is not fully accurate, Your Honor.' Mr. Whitfield stood. 'Your Honor, cookbooks are holiday gifts. People buy them no matter what. My client's mystery shelf dropped by half. One shelf is not the store.' The judge nodded. 'Counselor, your shelf only proves something if it represents the whole store. Can you show me that it does?'

Step 5 · Work the real question

Open PrepTest 110, Section 2, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Separate the scope of the theory (overall) from the scope of the evidence (one slice). The gap is whether the slice speaks for the whole. Negate each choice: if the slice were not typical, would the argument fall? Pick the one whose negation sinks it, and do not demand more than the argument needs.