Logical Reasoning · Sufficient Assumption

PrepTest 136 · Section 2 · Question 19

Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.

Step 1 · Pre-Question Guidance · read this first

Question type

Sufficient Assumption

The one idea it tests

Build a bridge from 'it belongs to category X' to 'anyone who treats it as Y got it wrong.'

The basic logic

The premise puts the thing into one category. The conclusion says reading it as a different kind of thing is a mistake. The new term in the conclusion is never linked to the premise. The magic key says: things in the first category cannot correctly be read as the second kind.

Tiny words that decide it

“since”
“thus”
“cannot”
“only if”

Playground logic

Kevin says his dinosaur book only lists facts, so it is a reference book, so the kids reading it as a story got it wrong. The patty is 'it's a reference book.' The top bun is 'reading it as a story is a mistake.' The missing sauce: a reference book cannot correctly be read as a story.

Step 2 · The kid story

Kevin's Book Is NOT a Story

Kevin held up his giant dinosaur book. "This book says it just lists facts about which dinosaurs lived when. So it's a reference book." Owen shrugged. "Tommy and I have been reading it like a bedtime story. We do voices." "Then you're doing it wrong!" Kevin cried. Priya tilted her head. "Hold on. You proved it's a reference book. But you never said a reference book can't be read as a story. Maybe it can be both." Kevin thought hard. "Okay. Reference books can never correctly be read as stories. There. Now I win." Lulu wrote on her clipboard: Add that rule and Kevin's conclusion clicks shut. Owen still did the T. rex voice anyway.

Step 3 · Try the model question

Kevin: My dinosaur book says it only lists facts about when each dinosaur lived, so it is a reference book. So Owen and Tommy, who read it as a bedtime story, have misunderstood it.

Which choice, if added as a premise, would make Kevin's conclusion airtight?

Step 4 · The courtroom tale

The Case of the Cookbook Novel

"Your Honor," said Mr. Whitfield, "the book in question calls itself a set of exact instructions for baking bread. That makes it a cookbook." Opposing counsel stood. "And my client reads it as a love story. What's wrong with that?" The judge turned to Mr. Whitfield. "You need one more brick, counselor." Mr. Whitfield bowed. "Then here it is: a cookbook can never correctly be read as a love story." "With that rule," the judge said, "your conclusion follows. Without it, you've only told me what shelf the book sits on, not how it may be read." Mr. Whitfield beamed. "That rule is my magic key, Your Honor."

Step 5 · Work the real question

Open PrepTest 136, Section 2, Question 19 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the premise and conclusion as two short lines. Circle the idea in the conclusion that never appeared in the premise. Build the bridge: 'anything in the premise's category cannot be correctly treated as the conclusion's kind.' Then test each choice with the Click Test: add it to the premises and ask whether the conclusion now has to be true. Skip choices that only confirm a premise, that point the wrong direction, or that talk about outcomes in the world.