Logical Reasoning · Sufficient Assumption

PrepTest 129 · Section 2 · 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

Sufficient Assumption

The one idea it tests

Supply the missing link that lets a counterexample knock down a 'must' rule.

The basic logic

The structure: Rule says every legitimate X must do Y. Then: if you count things made solely of Z as legitimate X, the rule falls. For that to follow with certainty, you need: things made solely of Z cannot do Y. Use the Click Test: add the link and the conclusion should snap shut.

Tiny words that decide it

“must”
“solely”
“if one regards”

Playground logic

Marcus's rule: every real dance must move you across the floor. Nadia: if freeze dance counts as a real dance, the rule is wrong, since freeze dance is nothing but still poses. The magic key: still poses can't move you across the floor. Click.

Step 2 · The kid story

The Freeze Dance Debate

At Lulu's dance party, Marcus made an announcement. "A real dance must have steps that move you across the floor. Otherwise it's just standing around." Nadia put her hands on her hips. "If you count freeze dance as a real dance, your rule is toast." "Why?" asked Marcus. "Freeze dance is only poses. You strike a pose, you hold it, you strike another one." Marcus scratched his head. "So?" Priya sipped her juice box. "Nadia's argument needs one more piece to click. She needs this: a dance made of nothing but still poses can't move you across the floor. If that's true, and freeze dance is a real dance, then Marcus's rule breaks." Lulu wrote it down. Biscuit struck a pose and held it, which was suspicious.

Step 3 · Try the model question

Marcus says that a real dance must have steps that move you across the floor. But that claim must be rejected if one counts freeze dance as a real dance, because freeze dance consists solely of still poses.

Which one of these, if assumed, allows Nadia's conclusion to follow?

Step 4 · The courtroom tale

The Case of the Silent Alarm Clock

Mr. Whitfield faced the jury. "The manufacturer claims every real alarm clock must make a sound to wake you." He held up a small box. "But this clock consists solely of a light that slowly brightens. If you count it as a real alarm clock, the manufacturer's rule fails." The judge leaned in. "Counselor, your argument is missing something." "Yes, Your Honor. I must add that a clock made only of a brightening light makes no sound." "Ah. Then if it is a real alarm clock, here is a real alarm clock that makes no sound." "And the rule falls, Your Honor. Click."

Step 5 · Work the real question

Open PrepTest 129, Section 2, Question 7 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the rule as 'every legitimate X must do Y.' Write the counterexample as 'things consisting solely of Z are legitimate X.' The missing link is the bridge between Z and not-Y. Look for the choice that guarantees things made of Z cannot do Y. Add it to the argument and see if the conclusion is forced. Reject choices that are about popularity, the speaker, or broad claims that do not connect Z to Y.