Logical Reasoning · Flaw

PrepTest 148 · Section 1 · Question 13

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

Step 1 · Pre-Question Guidance · read this first

Question type

Flaw

The one idea it tests

If A guarantees B, that does not mean B needs A. Losing one route to a result does not rule the result out.

The basic logic

Spot an if-then in the patty. Then watch the top bun flip only the front half to 'not' and conclude 'not' about the back half. That is the Flipped Switch: a sufficient condition treated as if it were the only door in.

Tiny words that decide it

“if”
“hence”
“not”
“then”

Playground logic

Kevin's patty: if he studies his flashcards, he passes. His top bun: he did not study, so he will not pass. He treated one way of passing as the only way, which is the Flipped Switch.

Step 2 · The kid story

Kevin's Flashcard Logic

The dinosaur quiz was on Friday. On Thursday night, Kevin flopped on the couch. "Okay, here's the deal," he told Tommy. "If I study my flashcards, I'll pass the quiz. I didn't study my flashcards, because I was building a pineapple pizza volcano. So I won't pass. So I might as well not even try." Tommy blinked. "Kevin, you know more about dinosaurs than anyone in the school. You talk about them during lunch, during recess, during the fire drill." Lulu looked up from her clipboard. "Flashcards are one way to pass. You said 'if.' You didn't say 'only if.'" Kevin sat up slowly. "Wait. So I could still pass?" Priya nodded. "Studying was a ticket in. Not the only ticket."

Step 3 · Try the model question

Kevin reasons: "If I study my flashcards, I will pass the dinosaur quiz. I did not study my flashcards. So I will not pass the quiz."

Kevin's reasoning is weakest for which reason below?

Step 4 · The courtroom tale

The Key and the Window

A hardware store was missing a ladder. The prosecutor pointed at the shopkeeper's nephew. Then, surprisingly, the nephew's own lawyer argued a strange way. "Your Honor, if my client had a key, he could get into the store. My client had no key. Hence he never got in." Mr. Whitfield, watching from the gallery, could not help himself. He leaned over the rail. "Pardon me, Your Honor. A key is one way in. The back window was open all week." Judge Okafor sighed. "Mr. Whitfield is not on this case, but he is correct. Counsel, a key would have been enough to get in. You have not shown it was required. Find a better argument."

Step 5 · Work the real question

Open PrepTest 148, Section 1, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Turn each premise into an if-then statement and mark which part is the trigger and which is the result. Then see what the conclusion does. If it says the trigger is absent and therefore the result is absent (or there is nothing left), the argument has treated a trigger as the only door. Pick the description that names confusing a condition that is enough with one that is required. Ignore descriptions of flaws that the argument never commits.