PrepTest 138 · Section 4 · Question 21 (Parallel Flaw)

Step 1 · Pre-Question Guidance · read this first

Question type

Parallel Flaw

The one idea it tests

'Most of a big group' does not mean 'most of a smaller group inside it.'

The basic logic

X-Ray the shape: every A is a B; most B are C; every C is D; so most A are D. The flaw sits in the middle link. The A's could all be in the small slice of B's that are not C. Find the choice with the same skeleton and the same broken link.

Tiny words that decide it

“each”
“most”
“no”
“so”

Playground logic

Every kid in Kevin's dinosaur club is a fourth grader, most fourth graders ride the bus, and every bus rider knows the bus song, so Kevin says most club kids know the song. The flaw: the club might be made entirely of the few fourth graders who walk to school. 'Most of the grade' says nothing about 'most of the club.'

Step 2 · The kid story

Kevin and the Bus Song

Kevin bounced in his seat. "Everybody in my dinosaur club is a fourth grader. Most fourth graders ride the bus. And everybody who rides the bus knows the bus song. So most of my dinosaur club knows the bus song!" Lulu tapped her clipboard. "Who's in your club?" Kevin counted. "Me, Tommy, and the twins who live across the street from school." Lulu looked up. "You all walk to school." Kevin gasped. "We do!" Priya sipped her juice box. "Most of the whole grade rides the bus. That doesn't tell us about your little club. Your club might be made of the walkers." Kevin thought for a second. "Well, at least we know the dinosaur song."

Step 3 · Try the model question

Every kid in the dinosaur club is a fourth grader. Most fourth graders ride the bus. Everyone who rides the bus knows the bus song. So most kids in the dinosaur club know the bus song.

Which argument below has a flaw most like the one in Kevin's argument?

Step 4 · The courtroom tale

The Orchard Inspector

Judge Abernathy listened as a fruit inspector testified. "Every apple from the north orchard is a red apple. Most red apples are sweet. Every sweet apple passes our taste test. So most north-orchard apples pass." Mr. Whitfield adjusted his suspenders. "Your Honor, most red apples in the whole state may be sweet. But the north orchard might grow only the sour red kind." The inspector blinked. "I never checked which red apples grow there." Mr. Whitfield turned to the bench. "A fact about most of the big basket says nothing about most of one small bucket." The judge nodded. "The inspector's chain has a weak link in the middle. Claim denied."

Step 5 · Work the real question

Open PrepTest 138, Section 4, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

X-Ray the stimulus first: write each premise as all, most or none, in order, and note the conclusion's quantifier. Spot the bad link: a 'most' about a big group being applied to a smaller group inside it. Then X-Ray each choice the same way. Keep only a choice whose links line up in strength and whose conclusion makes the same jump. Cross off choices that are valid, that end with a different quantifier, or that stack a different mix of links.