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Logical Reasoning · Parallel Flaw

PrepTest 142 · Section 2 · Question 23

Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.

Step 1 · Pre-Question Guidance · read this first

Question type
Parallel Flaw (flaw by counterexample)
The one idea it tests
Show a bad shape is bad by pouring obviously true premises into it and getting an obviously false conclusion.
The basic logic
Map the shape: every A is a B; most B are C; so most A are C. The trouble: A could be a small slice of B that lands in the minority. The right answer keeps the same shape, uses premises we accept, and ends in something plainly silly.
Tiny words that decide it
  • “every”
  • “most”
  • “all”
  • “so”

Playground logic

Marcus says every dog at the park is an animal, and most animals at the park are pigeons, so most dogs at the park are pigeons. Same skeleton as the real argument, and the silly top bun proves the skeleton is broken. That is the X-Ray Method used as a weapon.

Step 2 · The kid story

Marcus and the Pigeon Dogs

Marcus was proud of a new proof. "Every sticker in Lulu's album is shiny. Most shiny stickers at the shop are star stickers. So most stickers in Lulu's album are stars!" Lulu opened her album. Almost all her shiny stickers were dinosaurs. Kevin cheered. Priya set down her juice box. "Let me show the problem with the same pattern. Every dog at the park is an animal. Most animals at the park are pigeons. So..." "Most dogs at the park are pigeons," Marcus finished, then froze. Biscuit barked, clearly not a pigeon. "The dogs are a small slice of the animals," Priya explained. "A small slice can sit entirely outside the 'most.' Same with Lulu's dinosaurs." Marcus sighed. "My proof just turned Biscuit into a bird."

Step 3 · Try the model question

Every sticker in Lulu's album is shiny. Most of the shiny stickers sold at the shop are star stickers. So most of the stickers in Lulu's album are star stickers.

Which choice best exposes the flaw by using the same reasoning to reach a plainly silly result?

Choices

Step 4 · The courtroom tale

The Case of the Marching Tubas

The prosecutor argued: "Every tuba player in the parade marched in the band. Most band members wore red hats. So most tuba players wore red hats." Mr. Whitfield rose. "Your Honor, by the same logic: every judge in this building is a person. Most people in this building are jurors. So most judges here are jurors." Judge Reed laughed. "I assure you I am not a juror." "Exactly," said Mr. Whitfield. "True premises, silly ending. The tubas might be the few who wore blue." The judge nodded. "The shape is broken. Bring me a photo of the hats."

Step 5 · Work the real question

Open PrepTest 142, Section 2, Question 23 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the original as a skeleton with letters: every A is B, most B are C, so most A are C. For each choice, write its skeleton the same way, ignoring the topic. Keep only choices whose skeleton matches exactly, in the same order of quantity words. Then check that the premises sound true and the conclusion sounds clearly false; that is the one that proves the flaw.