Logical Reasoning · Necessary Assumption

PrepTest 140 · Section 2 · Question 6

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

Evidence about a subgroup supports a conclusion about 'most' of the whole group only if the subgroup is most of the group.

The basic logic

The premises describe what happens to people who do something over and over. The conclusion is about most people who do it at all. The secret sauce: most of them are repeaters. Pull the Plank: if most did it only once, the conclusion falls.

Tiny words that decide it

“most”
“repeatedly”
“eventually”

Playground logic

Priya's patty is about kids who fly kites again and again: the more flights, the better the chance one hits the oak. Her top bun is about most kite fliers. The plank holding it up: most kite fliers fly over and over.

Step 2 · The kid story

The Kite-Eating Oak

Behind the school stands a huge oak tree that loves to eat kites. Priya studied it like a scientist. 'Kids who fly kites over and over without trouble get bolder,' she said. 'Each good flight makes them think the tree can't get them. But the more times you fly, the bigger the chance that one flight ends in the branches. So most kids who fly kites here will eventually lose one to the oak.' Lulu raised her pencil. 'Wait. That only works if most kite fliers fly a lot. What if most kids fly once and quit?' Priya paused mid-sip. 'Then my rule only covers the regulars.' Tommy, who had flown once and lost his kite immediately, quietly ate a lollipop.

Step 3 · Try the model question

Kids who fly kites over and over without trouble get bolder. But the more times someone flies, the bigger the odds that one flight ends in the oak. So most kids who fly kites here will eventually lose one to the oak.

Priya's argument depends on which assumption?

Step 4 · The courtroom tale

The Library Fine Case

The library sued a patron over late fees. 'People who return books late again and again stop worrying,' said its lawyer. 'But every late return raises the chance of being flagged. So most late returners will eventually be flagged.' Mr. Whitfield stood, suspenders creaking. 'Your Honor, that only works if most late returners do it repeatedly. My client was late exactly once, as were most people on this list.' Judge Romero peered at the list. 'Counsel, your argument is about the regulars. Your conclusion is about everyone. The bridge between them is missing.'

Step 5 · Work the real question

Open PrepTest 140, Section 2, Question 6 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Locate the conclusion and note its group and quantity word. Then note which group the premises actually describe. If the premises describe only a slice, the needed assumption probably says that slice is big enough to carry the conclusion. Test each choice with Pull the Plank: negate it and see if the argument breaks. Ignore choices about feelings or traits the argument mentions but does not need.