PrepTest 146 · Section 1 · Question 21 (Parallel Flaw)

Step 1 · Pre-Question Guidance · read this first

Question type

Parallel Flaw

The one idea it tests

Match an argument that jumps from 'every item has A or B' to 'some items have A and some have B.'

The basic logic

Premises: most items have feature one; nearly all items have feature two or feature three, or both. Conclusion: some items have one and two, and some have one and three. Flaw: the 'or' could all land on one side, so only one of those pairs is guaranteed. Find a choice with the same quantities, the same 'or', and the same overreaching 'and'.

Tiny words that decide it

“most”
“virtually every”
“or both”
“some ... and some”

Playground logic

Marcus's patty: most lunchboxes have an apple, and nearly all have a sandwich or a wrap, or both. His top bun: some have apple plus sandwich, and some have apple plus wrap. The flaw: every lunchbox might hold sandwiches and no wraps, so he only earned one of those pairs.

Step 2 · The kid story

Marcus Inspects the Lunchboxes

Marcus appoints himself Lunchbox Detective. 'Fact one,' he announces, 'most lunchboxes in our class have an apple. Fact two: almost every lunchbox has a sandwich or a wrap, or both. Therefore, case closed: some lunchboxes have an apple and a sandwich, and some have an apple and a wrap.' Priya sets down her juice box. 'Marcus, what if nobody brought a wrap today? Then almost every lunchbox has a sandwich, and the or-part is still true.' Marcus blinks. 'Then... no apple-and-wrap lunchboxes.' 'Right,' says Priya. 'You only proved apple plus sandwich-or-wrap. You can't promise both kinds of pairs.' Lulu writes on her clipboard: Most A. Nearly all B or C. So some A and B, and some A and C. Broken.

Step 3 · Try the model question

Marcus: Most lunchboxes in our class have an apple. Nearly every lunchbox has a sandwich or a wrap, or both. So there must be some lunchboxes with an apple and a sandwich and some with an apple and a wrap.

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

Step 4 · The courtroom tale

The Case of the Garden Plots

Your Honor, the plaintiff argued that most garden plots grow tomatoes, and nearly every plot grows beans or peas, or both. 'So,' he said, 'some plots grow tomatoes with beans and some grow tomatoes with peas.' Mr. Whitfield rose. 'Your Honor, what if no one planted peas this year? Every plot could grow beans, and his or-statement still holds. He has proved tomatoes with beans or peas, not both pairings.' The judge sighed. 'An or does not promise you an and. Next witness.'

Step 5 · Work the real question

Open PrepTest 146, Section 1, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

First X-ray the stimulus: write its quantities (most, nearly all), its either-or premise, and its two-part conclusion. Name the flaw in plain words. Then check each choice for the same skeleton: same strength words, same or-premise, same two-part conclusion that the or cannot guarantee. Reject choices whose conclusion follows validly, whose premises are built differently, or whose reasoning breaks in another place. Order of sentences can be shuffled; bones must match.