Logical Reasoning · Must Be True

PrepTest 116 · Section 3 · Question 7

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

Step 1 · Pre-Question Guidance · read this first

Question type

Must Be True

The one idea it tests

Two linked conditional statements form a chain. The safe inference is the contrapositive of the whole chain.

The basic logic

Diagram the chain: lacking A leads to lacking B, and lacking B leads to lacking C. Flip and negate the whole thing: having C means having B, which means having A. Only an answer that follows the arrows (or the flipped, negated arrows) must be true.

Tiny words that decide it

“if”
“not”
“must”
“flip”

Playground logic

Lulu's chain: no library card means no reading room, and no reading room means no story time. Flip it: if you are at story time, you got into the reading room, so you have a library card.

Step 2 · The kid story

Lulu's Library Chain

Lulu made a sign for the library door. Rule one: no library card, no entry to the reading room. Rule two: if you can't get into the reading room, you can't join story time, because story time happens in there. Marcus read the sign. "So if I have a card, I get story time!" "Nope," said Lulu. "A card gets you past one door. It doesn't promise anything else." Tommy tried. "So if I'm not at story time, I don't have a card?" "Backwards Arrow," said Lulu. "Maybe you just went to the bathroom." Priya sipped her juice box. "Try it the safe way. Flip and negate. If you are sitting at story time, you got into the reading room. And if you got in, you have a card." Lulu gave her a gold star.

Step 3 · Try the model question

If you have no library card, you cannot enter the reading room. And if you cannot enter the reading room, you cannot join story time.

Given the two rules above, which one of the following must also be true?

Step 4 · The courtroom tale

The Pilot's License Puzzle

A flight school's rules said: no medical certificate, no solo flight. No solo flight, no license. A student pilot named Gus showed up holding a brand new license. The school's lawyer claimed he had cheated, since nobody remembered his checkup. Mr. Whitfield straightened his suspenders. "Your Honor, follow the rules backward, flipping each one. If Gus has a license, he flew solo. If he flew solo, he had a medical certificate. The rules themselves prove he had one." The judge checked the paperwork, and there it was, filed under a misspelled name. The lesson: flip and negate the whole chain, and the safe conclusion falls right out.

Step 5 · Work the real question

Open PrepTest 116, Section 3, Question 7 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Translate each sentence into an arrow: lacking the first thing leads to lacking the second; lacking the second leads to lacking the third. Link them into one chain. Write the contrapositive: having the last thing guarantees the middle thing, which guarantees the first. Hunt for the choice that matches one of these two directions. Cross out choices that reverse an arrow without negating, negate without reversing, or soften to probably.