Logical Reasoning · Necessary Assumption

PrepTest 157 · Section 3 · 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

The argument jumps from 'a guard's job is to stop harm' to 'calmer people need less guarding.' It must assume calm people cause less of that harm.

The basic logic

Line up the job (stop harm) and the new fact (most people behave a certain way). The conclusion only follows if that behavior actually links to less harm. Pull the Plank: if the behavior did not reduce harm, the conclusion collapses.

Tiny words that decide it

“purpose”
“hence”
“if most”
“less”

Playground logic

Owen's patty is that the recess monitor's job is to stop kids from knocking others down while chasing the swings. His top bun is that if most kids take turns, monitors need less power. The secret sauce: kids who take turns are less likely to knock others down. Pull that plank and the burger falls off the table.

Step 2 · The kid story

Fewer Whistles

Owen is class president for a week, and he has a plan. 'The recess monitors are there for one reason: to stop kids from knocking each other over while racing for the swings. So if most of us take turns, the monitors should not need so many whistles.' Lulu raises her clipboard. 'Wait. Does taking turns actually mean fewer kids get knocked over?' Owen blinks. 'Of course it does.' 'Then that's your hidden plank,' Priya says. 'If turn-takers knocked kids over just as often, the monitors would still have the same job to do.' Marcus suggests the monitors could just use quieter whistles. Nobody takes this seriously. Owen writes the assumption on the board in big letters.

Step 3 · Try the model question

Recess monitors exist to keep students from knocking each other down while racing for the swings. So if most students take turns, recess monitors should have less authority.

The argument depends on which one of the following assumptions?

Step 4 · The courtroom tale

The Hall Pass Hearing

Mr. Whitfield faced the school board, which was playing judge for the day. 'Your Honor, hall monitors exist to stop students from bumping into each other in the rush between bells. If most students walk on the right side, the monitors deserve fewer powers.' The board chair, a retired crossing guard, leaned forward. 'Counselor, your case needs one more brick. Do right-side walkers actually bump into people less?' Mr. Whitfield checked his notes. 'I assumed so.' 'You must. If right-side walkers bumped just as much, the monitors would be as needed as ever.' Mr. Whitfield added a page titled 'Fewer Bumps.'

Step 5 · Work the real question

Open PrepTest 157, Section 3, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the purpose premise and the conclusion side by side. Notice the new idea in the conclusion's 'if' clause that the premise never ties to the stated purpose. The needed assumption links that behavior to less of the harm the purpose names. Test each choice by negating it; the right one, once denied, breaks the argument. Watch for choices that are too strong.