Logical Reasoning · Flaw

PrepTest 128 · Section 2 · 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

Flaw

The one idea it tests

Claiming a mix beats each of its ingredients needs a test against each ingredient, not just one of them.

The basic logic

List every comparison in the conclusion. Then list the comparisons the evidence actually makes. The missing one is the flaw.

Tiny words that decide it

“better than”
“alone”
“or”

Playground logic

Marcus's top bun says the paper-and-ribbon tail beats paper alone AND ribbon alone. His patty only shows paper alone fails and the mix works. Ribbon alone never got tested: that's a Bad Comparison with a missing side.

Step 2 · The kid story

Marcus and the Kite Tail

Marcus held up his kite. "Science time. A tail made of paper strips is too light. My kite flipped over every time." Then he showed a new tail: paper strips tied together with ribbon. "With this mix, my kite flew steady as a rock!" He puffed out his chest. "And ribbon costs the same as paper. So everyone should use paper plus ribbon, not paper alone, and not ribbon alone." Priya tilted her head. "Did you ever fly the kite with a tail of just ribbon?" Marcus blinked. "Well, no." "Then how do you know the mix beats ribbon by itself?" asked Priya. "Maybe all ribbon works even better." Lulu wrote on her clipboard: Tested paper, tested mix. Ribbon alone: never tested. Marcus slowly lowered his kite. "Detective error," he mumbled.

Step 3 · Try the model question

Marcus: A kite tail of paper strips alone is too light to keep a kite steady. But a tail of paper and ribbon together kept my kite very steady. Ribbon costs about the same as paper. So kids would do better with a paper-and-ribbon tail than with a tail of paper alone or of ribbon alone.

Marcus's reasoning is weak because he offers no evidence that

Step 4 · The courtroom tale

The Blended Paint Claim

A paint company sued a rival over an ad. Their lawyer explained: "Plain chalk paint peels. Our chalk-and-clay blend does not peel. Clay costs the same as chalk. So our blend beats chalk paint and beats clay paint." Mr. Whitfield rose, thumbs in his suspenders. "Your Honor, I see a test of chalk alone. I see a test of the blend. Where is the test of clay alone?" The lawyer shuffled papers. "We, um, didn't run that one." The judge sighed. "Then you cannot claim the blend beats clay. You only beat one of your two opponents."

Step 5 · Work the real question

Open PrepTest 128, Section 2, Question 3 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write down the conclusion's comparisons: the mixture versus each single option it names. Then check the evidence: which of those comparisons was actually tested or supported? One side will have support and one side will be missing. The right answer names the comparison the argument needs but never backs up. Cross off choices naming things the argument already showed, or things it never claimed.