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Logical Reasoning · Principle (Apply)

PrepTest 148 · 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
Principle (Apply)
The one idea it tests
The principle gives a necessary condition: no requirement unless a certain risk. So the only safe conclusion it can justify is 'no requirement' when the risk is absent.
The basic logic
Translate 'No X unless R' into: requirement → risk. Contrapositive: no risk → no requirement. Look for an argument that shows the risk is missing and concludes there should be no requirement. Arguments that find a risk and conclude 'require it' run the arrow backwards.
Tiny words that decide it
  • “unless”
  • “no”
  • “should not”
  • “only if”

Playground logic

Ms. Rivera's rule: a chore needs a grown-up's sign-off only if doing it badly could break something valuable. Lulu shows plant watering can't break anything valuable, so no sign-off. That uses the rule the right way.

Step 2 · The kid story

The Sign-Off Rule

Ms. Rivera posted a new rule for classroom chores: "No chore needs a grown-up's sign-off unless doing it badly could break something valuable." Marcus raced to the board. "Washing the fish tank! If you mess it up, the tank could crack. So it needs a sign-off!" Priya tilted her head. "The rule only tells us when a sign-off is NOT needed, Marcus. It never says that danger is enough to require one." Lulu raised her clipboard. "Watering the plants. Even if you do it badly, nothing valuable can break. Some wet dirt, that's it. So watering plants should not need a sign-off." Ms. Rivera smiled. "Lulu used my rule exactly as written." Marcus sat down. "Unless," he muttered, "is a tricky word."

Step 3 · Try the model question

Principle: No chore should need a grown-up's sign-off unless doing it badly could realistically break something valuable.

Which conclusion below does the stated rule most clearly justify?

Choices

Step 4 · The courtroom tale

The Case of the Permit Clerk

A town law said: "No hobby needs a permit unless doing it carelessly could flood a neighbor's yard." A stamp collector was fined for having no permit. Mr. Whitfield stood up. "Your Honor, the law sets a gate. A permit is allowed only when there is real flood risk. Show me how a careless stamp collector floods anybody." The clerk shrugged. "Stamps are... dry." "Precisely. No flood risk, so under the law no permit can be demanded." Judge Lin nodded. "The law limits permits. It does not order them wherever there is risk. Fine dismissed."

Step 5 · Work the real question

Open PrepTest 148, Section 1, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Turn the principle into an arrow: requirement → condition. Write its contrapositive: condition missing → no requirement. Now scan the arguments for one whose premise says the condition is missing and whose conclusion says no requirement. Be wary of arguments that find the condition present and then demand the requirement; the principle never says the condition is enough. Also check that the premise truly denies the condition, not just part of it.