Logical Reasoning · Necessary Assumption

PrepTest 151 · Section 3 · Question 19

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

If a new rule is said to protect someone, the rule must actually change the behavior of somebody who was doing the harm.

The basic logic

The argument says a rule removes a motive, so a harm will stop. The gap: removing a motive only helps if at least somebody acts differently once it is gone. Pull the plank: if nobody would change, the protection is empty.

Tiny words that decide it

“would”
“remove”
“at least one”

Playground logic

Coach Rosa's new rule takes away the captain's reason to bench the little kids. Her top bun says the little kids will get protected. The secret sauce: at least one captain who would have benched them will now stop, or the rule protects nobody.

Step 2 · The kid story

The Bench Rule

On the playground soccer team, some captains bench the youngest kids so they can play every single minute themselves. Aunt Rosa, the volunteer coach, has a plan. "New rule," she says. "No captain may play more than twice the minutes of the kid who plays the least." Priya nods. "So if a captain benches the little kids, the captain gets benched too. No reason to do it anymore. The little kids are safe." Lulu raises her clipboard. "Only if the rule changes somebody," she says. "If every captain would keep benching anyway, nobody is protected." Marcus scoffs, then thinks. "So the plan needs at least one captain who stops benching because of the rule." Lulu smiles. "One is enough. Zero is not."

Step 3 · Try the model question

Some team captains bench the youngest players so the captains can play more minutes. A rule limiting each captain to twice the minutes of the least-used player would remove this reason for benching. So the rule would protect the youngest players from being benched.

The coach's argument depends on which one of the following?

Step 4 · The courtroom tale

The Parking Meter Plan

A town lawyer argued that a new meter rule would stop shop owners from hogging the street spots meant for shoppers. "The rule takes away their reason," she said. Mr. Whitfield rose, thumbs in his suspenders. "Your Honor, taking away a reason only matters if someone acts on it. If every owner would still hog spots for other reasons, the rule protects no shopper at all." The judge leaned forward. "So you need at least one owner who would change?" "Just one, Your Honor," said Mr. Whitfield. "Without that, the plan is a lock with no door." The lawyer scribbled a note to herself.

Step 5 · Work the real question

Open PrepTest 151, Section 3, Question 19 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Spot the conclusion: a rule would protect some group from a harm. Spot the premise: the rule removes one motive for the harm. Ask what must be true for removing a motive to protect anyone. Run the Pull the Plank test on each choice: negate it and see whether the protection vanishes. The right answer is modest, needing only that somebody would behave differently. Be wary of answers using all, never, or unless; they say too much.