Logical Reasoning · Necessary Assumption

PrepTest 124 · Section 3 · Question 8

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 harm needs an agent acting on an item, and the item merely turns up a lot in some place, the argument needs the agent to be present in at least some of those places.

The basic logic

Map the chain: agent plus item gives harm; item is often in place P; so changing P causes harm. The plank under it: at least some changed P actually have the agent working on the item. Pull that plank and the argument falls.

Tiny words that decide it

“often”
“at least some”
“when”

Playground logic

Marcus says turning art closets into reading nooks ruins books, because glue sticks ooze when a hot radiator warms them. His burger needs at least some of those nooks to really have a radiator warming glue. No radiator, no ooze.

Step 2 · The kid story

The Gooey Reading Nook

Marcus burst into class with news. 'When the old radiators heat up glue sticks, the glue melts and oozes over everything nearby. And leftover glue sticks pile up in art closets. So the school's habit of turning old art closets into reading nooks is ruining library books!' Ms. Rivera raised an eyebrow. Lulu flipped her clipboard. 'Wait. Do any of the closets that became nooks even have a radiator?' Marcus paused. Priya counted on her fingers. 'Room 4's nook has no radiator. Room 9's nook has no radiator.' Owen added, 'If none of them do, the glue just sits there being glue.' Marcus pulled his detective hat lower. 'Fine. I need at least one nook with a hot radiator and glue sticks, or my case melts.'

Step 3 · Try the model question

When hot radiators warm glue sticks, the glue oozes out and ruins nearby things. Art closets usually end up holding leftover glue sticks. So the school's habit of turning art closets into reading nooks is ruining library books.

Marcus's argument depends on which one of the following?

Step 4 · The courtroom tale

The Case of the Rusty Swing Set

A parents' group sued a town for turning old boat yards into playgrounds. Their lawyer said, 'Salt water rusts iron, and old iron chains are often left in boat yards. So these playgrounds are full of rust hazards.' Mr. Whitfield stood, thumbs hooked in his suspenders. 'Your Honor, the boat yards in question were dry yards, miles from the sea, and long since drained. Is there salt water at even one of these playgrounds?' The other lawyer flipped pages and found nothing. Judge Okafor tapped her gavel gently. 'Without salt water at any of them, the rust story has no engine.'

Step 5 · Work the real question

Open PrepTest 124, Section 3, Question 8 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Draw the chain: something acts on an item to cause harm, the item tends to turn up in a certain place, so changing that place causes harm. Look for the missing link: the agent must actually be at work in at least some of those changed places. Use the Pull the Plank test on each choice: negate it and see if the argument collapses. Prefer modest 'at least some' wording over broad rules the argument does not need.