Logical Reasoning · Flaw
PrepTest 137 · Section 2 · Question 9
Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.
Step 1 · Pre-Question Guidance · read this first
- “must”
- “only if”
- “to do X”
- “therefore”
Playground logic
Marcus's patty says you need a hall pass to get into the glitter closet. His top bun says Nadia has a pass, so she must have gone in. He turned a ticket you need into a ticket that makes you go: the Backwards Arrow.
Step 2 · The kid story
The Great Glitter Mystery
Somebody left a sparkly trail across the art room. Marcus put on his detective hat. "Clue one," he said. "Nobody gets into the glitter closet without a hall pass from Ms. Rivera. Clue two: Nadia has a hall pass in her pocket. Case closed. Nadia went into the glitter closet!" Priya sipped her juice box. "Marcus, a hall pass is something you need to get in. It doesn't push you through the door. Nadia used her pass to go to the nurse for a scraped knee." Lulu checked her clipboard. "Also, the glitter is purple, and Nadia hates purple." Marcus took off his hat. "Fine. But someone is getting glittered, and it's going to be me, isn't it?" It was. Biscuit had already rolled in it.
Step 3 · Try the model question
Marcus: No one can get into the glitter closet unless they have a hall pass from Ms. Rivera. Nadia has a hall pass from Ms. Rivera. So Nadia must have gone into the glitter closet.
Marcus's reasoning is faulty because he
Step 4 · The courtroom tale
The Case of the Missing Pie
"Your Honor," said the prosecutor, "only bakers with a kitchen key could reach the cooling rack. The defendant holds a kitchen key. Therefore she took the cherry pie!" Mr. Whitfield snapped his suspenders and stood. "Your Honor, a key is a requirement. It is not a confession. Twelve bakers carry that same key. Needing a key to reach the rack does not mean every key holder reached it." The judge nodded slowly. "So the key is a door you must pass through, not a road you must walk down." "Exactly," said Mr. Whitfield. "Show me crumbs, not keys." Case dismissed. The pie, sadly, was never found.
Step 5 · Work the real question
Open PrepTest 137, Section 2, Question 9 in your official copy from LSAC (for example in LawHub) and read it there.
Your answer
How to apply it
Find the rule in the patty and write it as an arrow: what must be true for something to happen. Then read the top bun and check which way it runs. If it starts from the required thing and lands on the happening, the arrow is backwards. Pick the answer that says a needed condition was treated as if it were enough. Skip answers naming flaws whose moves never appear in the argument.