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Logical Reasoning · Parallel Flaw

PrepTest 136 · Section 2 · Question 24

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

Step 1 · Pre-Question Guidance · read this first

Question type
Parallel Flaw
The one idea it tests
Copy the exact broken move: 'If A and B, then C. Not C. So not A.'
The basic logic
The argument says two things together would have produced a result. The result didn't happen. It then blames one of the two specific things. But the failure could have come from the other one. X-ray each choice for two joined conditions, a missing result, and a conclusion that picks out one condition.
Tiny words that decide it
  • “had”
  • “and”
  • “but”
  • “so”

Playground logic

Tommy says if Kevin had studied and slept well, he'd have aced the quiz. He didn't ace it, so Tommy concludes Kevin didn't study. But maybe Kevin studied and just didn't sleep. Two-ingredient recipe, failed dish, one ingredient blamed without proof.

Step 2 · The kid story

Kevin's Quiz Disaster

Kevin got a C on the dinosaur quiz. Tommy, chewing hidden candy, made a ruling. "If you'd studied hard and gotten a good night's sleep, you'd have aced it. You didn't ace it. So you didn't study." Kevin looked hurt. "I studied for three hours! I just stayed up late watching a volcano show." Priya set down her juice box. "Tommy, your rule needs both things. When the A didn't happen, all we know is that at least one was missing. You picked studying without proof." Lulu drew two boxes on her clipboard, labeled Study and Sleep, and put a question mark in each. "Either box could be the empty one," she said.

Step 3 · Try the model question

Tommy: If Kevin had studied hard and slept well, he would have aced the quiz. But he did not ace it. So Kevin did not study hard.

Which one of the following contains flawed reasoning most like Tommy's?

Choices

Step 4 · The courtroom tale

The Case of the Flat Cake

"Your Honor," said the bakery's lawyer, "if the baker had used fresh yeast and the right oven heat, the bread would have risen. It didn't rise. Therefore, the yeast was stale, and the supplier owes us money." Mr. Whitfield, representing the yeast supplier, rose. "Your Honor, the rule needs both yeast and heat. A flat loaf tells us at least one went wrong. It doesn't tell us which. Their own oven could be the culprit." The judge nodded. "Counsel, you've pinned the blame on one ingredient when the recipe had two suspects." The bakery's lawyer quietly checked the oven manual.

Step 5 · Work the real question

Open PrepTest 136, Section 2, Question 24 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Diagram the original: what two conditions are joined, what result they promise, what actually happened, and what the conclusion says. Name the flaw in one line. Then X-ray each choice, checking structure, not topic. Look for joined conditions, a failed result and a conclusion singling out one condition. Watch for choices that are valid, that use 'or' instead of 'and', or that run a different broken move.