Logical Reasoning · Flaw

PrepTest 151 · Section 3 · Question 9

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

Finding more of something only after you start looking harder does not prove there is more of it. Not having found it before is not proof it was not there.

The basic logic

Spot the shift: the evidence is about what people have discovered, but the conclusion is about what actually exists. Ask whether the old count was low only because nobody was searching. That is the No Footprints flaw: treating 'we never saw it' as 'it never happened.'

Tiny words that decide it

“until recently”
“must be”
“found”

Playground logic

Lulu only started checking under the bleachers last month, and now she keeps finding lost erasers. Marcus serves the patty (more erasers found since the search began) and slaps on a top bun (kids are losing more erasers than ever). The secret sauce is the No Footprints flaw: he assumes the erasers that nobody found before were never there.

Step 2 · The kid story

The Eraser Epidemic

Lulu got a new job: Lost and Found Captain. She grabbed her clipboard and started crawling under the gym bleachers every Friday. The first week she found three erasers. The next week, five. By the end of the month, she had a shoebox full of pink, blue and dinosaur-shaped erasers. Marcus put on his detective hat. "Case closed," he said. "Kids at this school are losing way more erasers than they used to. It's an eraser epidemic!" Priya sipped her juice box. "Marcus, before last month, did anyone ever look under the bleachers?" Marcus paused. "Well, no." "So those erasers might have been piling up for years. Nobody found them because nobody looked. Lulu didn't discover a new problem. She discovered an old pile."

Step 3 · Try the model question

Until last month, nobody at school thought many erasers got lost under the bleachers. Since Lulu began searching there every week, she has found dozens. So students must be losing more erasers now than they did before.

Marcus's reasoning is open to which criticism?

Step 4 · The courtroom tale

The Case of the Multiplying Pennies

Counselor Pratt paced. "Your Honor, before my client's neighbor bought a metal detector, she reported zero coins buried in the town park. Since then, she has dug up forty. Clearly park visitors are dropping more coins than ever, and my client's snack cart is to blame!" Mr. Whitfield snapped his suspenders. "One question for the witness. Before you bought that metal detector, did you ever search the park for buried coins?" The neighbor blinked. "Never. I didn't know they were there." "So the coins may have been there all along," said Mr. Whitfield. "Counsel has proved that the neighbor got a new gadget. He has not proved there are more coins. No footprints is not the same as no feet." The judge nodded. "Point taken, counselor."

Step 5 · Work the real question

Open PrepTest 151, Section 3, Question 9 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the conclusion and notice what kind of claim it is: a claim about how much of something really exists. Then look at the evidence and ask whether it is really about how much has been noticed or found. Check whether the amount of looking changed over time. The right answer will name the gap: treating a lack of earlier findings as proof that the thing was absent. Cross out choices that describe moves the argument never makes.