Logical Reasoning · Necessary Assumption

PrepTest 152 · Section 2 · Question 7

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 trigger based on a ratio between two things assumes the second thing stays put when the first one moves.

The basic logic

The rule: if A gets more than twice B, a move happens. Right now A is exactly twice B. The top bun: any rise in A causes the move. That only works if B does not also rise. Pull the Plank: if B rises too, the ratio may stay put and the conclusion falls.

Tiny words that decide it

“if”
“more than”
“at all”
“currently”

Playground logic

Priya will switch swings if her line gets more than twice as long as the other line. Right now it is exactly twice. Marcus says any new kid in her line means she switches, but that quietly assumes the other line does not grow too.

Step 2 · The kid story

The Swing Line Rule

Priya has a recess rule. 'If the line for the red swing gets more than twice as long as the line for the blue swing, I switch to blue.' Right now the red line has four kids and the blue line has two. Exactly twice. Marcus did the math out loud. 'So if even one more kid joins the red line, Priya switches. Guaranteed!' Kevin wandered up, humming a dinosaur song. He got in the red line. Five kids. Marcus cheered. But then Nadia and Owen hopped into the blue line. Now it had four. Five is not more than twice four. Priya stayed put and sipped her juice box. 'Marcus, your guarantee needed the blue line to stay the same. You never checked it.' Lulu wrote: 'Ratio rules have two sides.'

Step 3 · Try the model question

Priya will switch to the blue swing if the red swing line grows to over double the length of the blue swing line. The red line is now exactly double the blue line. So if the red line gets any longer, Priya will switch.

The conclusion above quietly relies on which assumption?

Step 4 · The courtroom tale

The Case of the Pie Price Promise

Lawyer Sal Ortega read a contract. 'Your Honor, Hilltop Diner promised to switch flour suppliers if its supplier ever charged more than double what the valley mill charges. Today the price is exactly double. So any increase at all forces the switch.' Mr. Whitfield tugged his suspenders. 'Unless the valley mill raises its price too, Your Honor. If both go up together, double stays double.' Mr. Ortega paused. 'I suppose my argument assumes the valley price holds steady.' The judge nodded. 'Pull that plank out and your conclusion sinks. A rule about one price compared to another has two moving parts. You only watched one.'

Step 5 · Work the real question

Open PrepTest 152, Section 2, Question 7 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Spot the trigger rule built on a comparison between two quantities, and note the current comparison. The conclusion claims that a change in one side alone flips the trigger. Ask what else could move and stop it. Pick the choice that keeps the other side steady, then Pull the Plank: negate it and see if the conclusion collapses. Ignore choices about motives, other expenses, or reasons for the outcome.