Logical Reasoning · Sufficient Assumption

PrepTest 151 · Section 3 · Question 4

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

If there are only two reasons something could be bad, you must knock out both to say it's fine.

The basic logic

The argument lists two doors to 'wrong' and closes one with evidence. The other door is left open. The magic key closes it, and then the conclusion clicks.

Tiny words that decide it

“only two”
“either”
“both”

Playground logic

Mom has only two reasons to ban snacks somewhere: crumbs on the rug, or Biscuit begging. Tommy shows Biscuit can't get to the treehouse, but forgets the rug. The magic key says there's no rug up there.

Step 2 · The kid story

Snacks in the Treehouse

Tommy wanted to eat crackers in the treehouse. "Mom only bans snacks for two reasons," he explained to Lulu. "Either crumbs get in the rug, or Biscuit begs and drools on everything." Lulu wrote it down. "So?" Tommy grinned. "So snacks in the treehouse are totally fine. Biscuit can't climb the ladder. He tried once and slid right back down." Biscuit sighed from the grass below. Lulu tapped her pencil. "You fixed the drool problem. What about the rug problem?" Tommy froze. "Uh. There's no rug up there. Just boards." Lulu smiled. "Now it clicks. Both reasons are gone." Tommy ate one cracker, and dropped two crumbs on the boards.

Step 3 · Try the model question

Mom bans snacks in a spot for only two reasons: crumbs ruining a rug, or Biscuit begging. So snacks in the treehouse are fine. After all, Biscuit cannot climb the treehouse ladder.

Tommy's conclusion is properly drawn if we add which assumption?

Step 4 · The courtroom tale

The Case of the Backyard Drum Set

In a fictional town, the rule said noise is only a problem for two reasons: it wakes babies, or it scares pets. Mr. Whitfield's client drummed in her shed. "Your Honor, there are no pets on this street. The animal shelter confirmed it." The judge peered over her glasses. "And the babies?" Mr. Whitfield pulled out a paper. "No babies live within earshot, Your Honor." The judge nodded. "Then both reasons are closed. Drum away." Mr. Whitfield smiled. "Without that paper, I'd have had only half a case."

Step 5 · Work the real question

Open PrepTest 151, Section 3, Question 4 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Underline the 'only two reasons' list. Then check which reason the evidence actually handles. Find the reason that is left untouched. The magic key will make that reason vanish in this new situation. Run the Click Test on each choice: add it to the premises and see whether the conclusion now has to follow. Choices that compare amounts, weigh costs, or talk about whether something can be done do not close the open door.