Logical Reasoning · Flaw

PrepTest 101 · Section 3 · Question 2

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

A comparison proves something only if the two things match in the exact respect the conclusion needs.

The basic logic

Spot the move 'A is like B; B suffers without something; so A needs our product.' Ask whether A and B are shown to be alike in the way that matters. If the only link is a surface likeness or a figure of speech, the flaw is a Bad Comparison.

Tiny words that decide it

“like”
“too”
“thus”
“should”

Playground logic

The Galactic Gum ad says a balloon wrinkles without air, and cheeks are round like a balloon, so cheeks need gum bubbles. Priya sees that roundness is the only thing they share, and it is not the part that matters, so the comparison is a Bad Comparison.

Step 2 · The kid story

The Balloon Cheeks Ad

A new commercial blares from the cafeteria TV. A Galactic Gum astronaut holds up a red balloon. "What happens to a balloon without air? It shrinks and wrinkles! Your cheeks are round like a balloon, so keep them puffed up the Galactic way: blow ten Galactic Gum bubbles a day!" Kevin pinches his cheeks in alarm. "Are my cheeks shrinking?" Priya sips her juice box. "Kevin, a balloon is a rubber bag. If you let the air out, nothing's left inside to hold its shape. Your cheeks are full of muscle and fat and bone. They don't stay round because of air." Marcus frowns. "But they're both round." "Being round is the only thing they share," Priya says. "And that's not the part that matters."

Step 3 · Try the model question

A gum ad says that a balloon without air shrinks and wrinkles. It then concludes that because cheeks are round like a balloon, kids should blow Galactic Gum bubbles every day to keep their cheeks round.

The reasoning in the ad is most open to which criticism?

Step 4 · The courtroom tale

The Leaky Bridge

A city wants to fine a homeowner for not sealing his wooden deck. The city's lawyer, Ms. Petrov, waves a photo. "Look at the old Fulton Bridge. Its steel rusted and failed without a sealing coat. A deck is a structure, just like a bridge. So this deck must be sealed!" Mr. Whitfield stands, thumbs in his suspenders. "Your Honor, steel rusts. Wood doesn't. The bridge needed a coat because water attacks steel in a particular way. My client's deck is made of cedar, which handles rain on its own for decades. Calling them both structures doesn't make them alike in the way that counts." The judge nods. "A comparison has to match on the point it's meant to prove."

Step 5 · Work the real question

Open PrepTest 101, Section 3, Question 2 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the top bun (what the speaker wants you to do) and the patty (a story about some other thing). Ask: what link joins the two? If the link is only a nickname or a shared look, pre-phrase 'the two things are not alike in the way that counts.' Read the choices for that idea. Cross off Phantom Pieces that describe moves the argument never makes, such as a hidden cause or a word with two meanings.