Logical Reasoning · Parallel Flaw
PrepTest 103 · Section 1 · Question 21
Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.
Step 1 · Pre-Question Guidance · read this first
- “not all”
- “every”
- “must be”
Playground logic
Marcus's patty: not all shiny stickers smell. Lulu's book is all shiny stickers. His top bun: so some of Lulu's stickers don't smell. The leak: Lulu might have picked only the shiny ones that do smell.
Step 2 · The kid story
The Scratch-and-Sniff Sting
Marcus inspected Lulu's sticker book with a magnifying glass. "Fact one," he said. "Not every shiny sticker is scratch-and-sniff. I have a shiny dinosaur that smells like nothing." "That's Kevin's dinosaur," said Tommy. "Fact two," Marcus went on. "Every sticker in Lulu's book is shiny. So, case closed: some of Lulu's stickers must not smell." Lulu held up her clipboard. "I only collect shiny stickers that smell. That is my whole rule. I sniff before I trade." Marcus scratched a cupcake sticker. Cupcake. He scratched a rocket. Somehow, also cupcake. He scratched a frog. Grape. "Okay," he said. "Some shiny stickers don't smell. But that doesn't mean Lulu's don't." "Her book could be all the smelly ones," Priya said. Marcus quietly put his magnifying glass away.
Step 3 · Try the model question
Not every shiny sticker is scratch-and-sniff. Every sticker in Lulu's book is shiny. So some of the stickers in Lulu's book must not be scratch-and-sniff.
Which argument has the same faulty pattern as Marcus's?
Step 4 · The courtroom tale
The Case of the Grumpy Goats
At the county fair hearing, Mr. Whitfield argued against a petting zoo. "Not all farm goats are gentle. Every goat in this petting zoo came from a farm. Therefore, some of these goats must be ill-tempered." The zoo's owner raised her hand. "Your Honor, we picked only the gentle ones. We tested each goat with a toddler and a carrot." Judge Fairweather nodded. "Counselor, a mixed barn does not make a mixed petting zoo. A small group drawn from a big mixed group can still be all one kind." Mr. Whitfield looked at the goat chewing his tie. "This one is gentle," the owner said. "It just likes ties." The petition was denied.
Step 5 · Work the real question
Open PrepTest 103, Section 1, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.
Your answer
How to apply it
Translate the argument into letters: some A are not B; every C is an A; so some C are not B. Write that skeleton in your margin. Then translate each choice the same way, slot by slot, checking the quantifiers (not all, every, some). Throw out choices that are valid, that reverse a premise, or that use different quantifiers. The right one makes the same leap: assuming a subgroup must share the mix of the larger group.