Logical Reasoning · Necessary Assumption

PrepTest 110 · Section 3 · Question 3

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

The evidence covers one group, but the conclusion is about most of the whole; the argument needs that group to be most of the whole.

The basic logic

Spot the group the premises protect and the group the conclusion talks about. If the conclusion says most of the total is fine, the protected group must make up most of the total. Pull the Plank: if that group were only a small slice, the conclusion collapses.

Tiny words that decide it

“most”
“therefore”
“thus”
“only”

Playground logic

Marcus proves the big sticker boxes are safe in the locked closet, then announces most of the school's stickers are safe. His secret sauce is that the big boxes hold most of the stickers. Pull that plank and the burger falls apart.

Step 2 · The kid story

Biscuit and the Sticker Heist

Biscuit had a new hobby: chewing stickers. Any sticker sheet left in a desk pile in the hallway was a goner. "We can't keep sticker sheets in the hallway," Lulu said. "Biscuit gets them." Marcus puffed out his chest. "No worries. The big sticker boxes are in the locked supply closet, and Biscuit can't open doors. So most of the school's stickers are safe." Priya sipped her juice box. "That works only if the big boxes hold most of the stickers." Lulu flipped her clipboard. "Ms. Rivera has three hundred sheets in her desk drawer. And the closet boxes hold maybe fifty." Marcus blinked. "Then most of the stickers are in the drawer." Biscuit wagged his tail and looked at the drawer.

Step 3 · Try the model question

Biscuit chews any sticker sheets left in the hallway, so stickers cannot be kept there. Luckily, the big sticker boxes are kept in the locked supply closet, which Biscuit cannot open. Therefore, most of the school's stickers are safe from Biscuit.

The argument relies on which one of these assumptions?

Step 4 · The courtroom tale

The Case of the Flooded Basement

A storage company was sued after a flood. Its lawyer was calm. "Your Honor, the water only reached the ground-floor lockers. Our large vault is on the third floor, safe and dry. Therefore most of our customers' belongings are safe." Mr. Whitfield stood and snapped his suspenders. "One question. How much is stored in the vault?" The lawyer hesitated. "Some furniture." Mr. Whitfield held up a list. "Two hundred ground-floor lockers, packed full. One vault, half empty. My friend's argument needs the vault to hold most of the goods. It does not." The judge sighed. "Counsel, a safe vault means little if the treasure was in the basement."

Step 5 · Work the real question

Open PrepTest 110, Section 3, Question 3 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Identify the conclusion's scope (how much of the total is safe or fine) and the evidence's scope (which part is protected). Notice the gap: the evidence is about one part, the conclusion is about most of everything. Look for the choice that links them by saying the protected part is the big share. Confirm with the Pull the Plank test: deny that choice and see whether the conclusion falls.