Logical Reasoning · Must Be True

PrepTest 107 · 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

Question type

Must Be True (Inference)

The one idea it tests

When one speaker's claims are lined up, they may contradict each other.

The basic logic

Someone says a danger is zero. The same someone defends an action by pointing to a threat that, by their own admitted chain of 'only if' steps, can exist only if the danger is real. Put the chain together and you see their statements could never all hold at once.

Tiny words that decide it

“only if”
“even”
“admits”

Playground logic

Tommy says the ladder is perfectly safe, yet he says he needs a pillow pile to stop Mom's ban, and Mom bans only if someone falls. So his own words clash. The answer is not that the ladder is unsafe; it is that Tommy contradicts himself.

Step 2 · The kid story

The Pillow Pile

Tommy built a ladder up to the tree house. "It's totally safe," he told Lulu. "No one could ever fall off." Then he spent an hour dragging every couch pillow under the ladder. "Why the pillows?" asked Lulu. "So Mom doesn't ban the tree house," Tommy said. "She'll ban it only if somebody gets hurt. And somebody can get hurt only if they fall off the ladder." Lulu wrote it all down. Then she read it back. "Ladder can't be fallen off. Pillows needed because Mom might ban it. Ban only if someone's hurt. Hurt only if someone falls." She looked up. "Tommy, those can't all be true together." Priya nodded. "We don't know if the ladder is safe. We just know Tommy's story doesn't fit together." Tommy hugged a pillow. "It's a very comfy contradiction."

Step 3 · Try the model question

Tommy says his tree house ladder is perfectly safe, so Lulu's worry about falls is silly. He also says he piled pillows under it to keep Mom from banning the tree house. But Tommy agrees that Mom would ban it only if someone got hurt, and that someone could get hurt only by falling off the ladder.

If Tommy really holds all of these views, which one of the following must be true?

Step 4 · The courtroom tale

The Case of the Unbreakable Vase

A moving company swore under oath that its packing was flawless and no vase it packed could ever break. Mr. Whitfield held up a paper. "Your Honor, this same company bought special insurance last month. Its manager testified the insurance protects it from customer lawsuits. He also agreed a customer can sue only if an item breaks, and an item can break only if the packing fails." He let that sink in. "So the company says the packing never fails, yet it pays to protect itself from what happens only when the packing fails. I am not saying which claim is false. I am saying they cannot both be true." The judge wrote one word on her pad: inconsistent.

Step 5 · Work the real question

Open PrepTest 107, Section 1, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

List each claim the speaker reports about one party. Turn the 'only if' pieces into arrows and chain them. Then compare the end of the chain with the party's opening claim. If they clash, the safest answer says the position does not hang together, not which half is false. Avoid choices that decide which claim is wrong, guess at hidden motives, or reach beyond what the party actually said.