Logical Reasoning · Must Be True

PrepTest 154 · Section 2 · Question 22

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

Step 1 · Pre-Question Guidance · read this first

Question type

Inference / Must Be True

The one idea it tests

Combine 'all of group X has trait T' with 'few of group Y have trait T' to learn that most of Y is not X.

The basic logic

Translate each statement into all/most/few form. Chain them: anything in X has T, and most of Y lacks T, so most of Y cannot be in X. Ignore statements that do not connect.

Tiny words that decide it

“all”
“few”
“most”
“if not all”

Playground logic

Owen's lunchbox holds two key facts: every red marble is shiny, and few big marbles are shiny. Put them together and most big marbles must lack shine, so most big marbles cannot be red. The Spoiler Test: no story keeps both facts true and makes that false.

Step 2 · The kid story

Owen's Marble Census

Owen dumps his marble jar onto the rug, and Lulu grabs her clipboard. "Every single one of my red marbles is shiny," Owen says proudly. "But only a few of my big marbles are shiny. Most big ones are dull and scratched." Biscuit sniffs a marble and sneezes. Marcus jumps in. "Then most of your shiny marbles are red!" Priya shakes her head. "Nope, that is the arrow backwards. Think about the big ones. If a marble is red, it is shiny. Most big marbles are not shiny. So most big marbles cannot be red." Lulu writes: 'All red are shiny. Few big are shiny. So most big are not red.' Owen counts to check. It works, of course.

Step 3 · Try the model question

Owen: All of my red marbles are shiny. Only a few of my big marbles are shiny.

Assuming everything Owen says is correct, which choice has to hold?

Step 4 · The courtroom tale

The Case of the Muddy Boots

Mr. Whitfield addresses the jury. "Fact one: every boot from the Hillcrest factory is waterproof. Fact two: only a few of the boots seized in this warehouse are waterproof." He snaps his suspenders. "So, ladies and gentlemen, most of these seized boots cannot be from Hillcrest." Opposing counsel objects. "Then most waterproof boots are from Hillcrest!" The judge sighs. "Counsel, you flipped the arrow. Mr. Whitfield simply combined an 'all' with a 'few.' His conclusion has to be true if his facts are." The jury nods. Lulu, visiting on a field trip, whispers, "Lunchbox Rule."

Step 5 · Work the real question

Open PrepTest 154, Section 2, Question 22 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write each statement as a quantity claim: all, most, few. Find the two that share a trait and can be chained. An 'all X are T' plus 'few Y are T' gives 'most Y are not X.' Set aside statements that never link. Then Spoiler Test each choice: can you build a world where every premise holds but the choice fails? If yes, cut it. Watch for reversed arrows and choices that mix unrelated groups.