Logical Reasoning · Necessary Assumption

PrepTest 102 · Section 3 · Question 2

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 you block a cause of most problems, the argument needs those problems not to show up anyway by some other route.

The basic logic

The argument says a factor is involved in most cases of a problem, then says removing the factor prevents most cases. The secret sauce: without the factor, the problem would not simply happen anyway. Pull the Plank on each choice: if negating it breaks the argument, it is the one.

Tiny words that decide it

“most”
“a factor in”
“prevent”
“all”

Playground logic

Priya notices that most juice spills happen with cups filled to the brim, so she wants lids on every cup. Her top bun says lids will stop most spills. The secret sauce is that, once the brim problem is gone, the spills would not just happen some other way.

Step 2 · The kid story

Priya's Great Lid Plan

Lunch at school was a splash zone. Lulu counted on her clipboard: most spills happened when someone carried a cup filled right to the top. Priya set down her juice box. "Easy fix. Put a lid on every cup. Lids stop sloshing, so lids stop most spills." Marcus squinted. "What if kids just knock the whole cup over instead?" Priya thought. "Good point. My plan only works if, with the sloshing gone, the cups would not mostly end up spilled some other way. If a lidded cup still tips and pops open half the time, the lids do not help much." Kevin raised a hand. "Can the lids have dinosaurs on them?" Priya smiled. "Sure. But the plan still needs that hidden plank."

Step 3 · Try the model question

Most lunchroom spills involve cups filled to the brim. So putting a secure lid on every cup would prevent most lunchroom spills.

Priya's plan depends on which assumption?

Step 4 · The courtroom tale

The Case of the Sneaky Scarecrow

A farmer sued a scarecrow maker. "You promised your scarecrows would stop most crop damage, because birds cause most of it." The maker's lawyer said, "And birds did stay away!" Mr. Whitfield, for the farmer, adjusted his suspenders. "True. But with the birds gone, rabbits ate the same crops. Your promise quietly assumed that, once birds were blocked, the damage would not simply arrive another way. That assumption was false." The judge tapped her pen. "A plan to stop most harm by blocking one cause needs the harm not to sneak in through another door. Judgment for the farmer."

Step 5 · Work the real question

Open PrepTest 102, Section 3, Question 2 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Mark the conclusion: removing some factor will prevent most of a problem. Mark the evidence: the factor is involved in most cases. Ask what must be true for the jump to work. Look for an answer saying the problem would not happen anyway once the factor is gone. Negate each contender. The right one, when negated, leaves the problem still happening, and the conclusion collapses.