Logical Reasoning · Flaw

PrepTest 141 · Section 2 · 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

A number staying flat does not prove a measure failed: without the measure, the number might have climbed.

The basic logic

The argument says a rising effort brought no drop, so the effort cannot help. Ask: compared to what? The flaw is ignoring the world where the effort never happened.

Tiny words that decide it

“cannot”
“no reduction”
“but”
“demonstrates”

Playground logic

Marcus says adding more recess monitors cannot cut scraped knees (top bun), because monitors went up and scrapes stayed flat (patty). He forgets the what-if world: without the extra monitors, scrapes might have shot up. That is a Bad Comparison: he compares to the past, not to the no-monitor path.

Step 2 · The kid story

The Scraped Knee Report

Marcus marched into the nurse's office holding a chart. "Behold! Five years ago, our school had two recess monitors. Now we have eight. The number of scraped knees per year? Exactly the same. Monitors are useless!" Nadia looked over his shoulder. "Also, five years ago, we didn't have the new climbing wall, the zipline, or the extreme tag league." "So?" said Marcus. Priya sipped her juice box. "So maybe scrapes would have tripled without the extra monitors. Flat might be a win." Marcus squinted at the chart. "You're saying the monitors could be holding back a scrape flood?" "I'm saying your chart can't rule it out," said Priya. Owen limped past with a fresh bandage. "Zipline," he explained.

Step 3 · Try the model question

Marcus: The school's yearly count of scraped knees has not dropped over the last five years. Yet the number of recess monitors has quadrupled, and the cost of paying them keeps rising. This proves that adding monitors does nothing to reduce scraped knees.

The reasoning in Marcus's argument is flawed because it

Step 4 · The courtroom tale

The Lighthouse That Did Nothing

Mr. Whitfield argued for cutting the harbor's new lighthouse budget. "Your Honor, the town built three extra lighthouses over ten years. Shipwrecks per year? No change at all. Clearly lighthouses do nothing!" The harbor master's lawyer stood. "Your Honor, in those same ten years, harbor traffic doubled and storms got worse. Without the lighthouses, wrecks might have soared." Judge Ruiz nodded. "Mr. Whitfield, flat numbers do not show the lighthouses failed. You have not told me what would have happened without them." Mr. Whitfield tugged his suspenders. "I suppose I compared the harbor to its past, not to a harbor with no new lights." "Budget stays," said the judge.

Step 5 · Work the real question

Open PrepTest 141, Section 2, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the conclusion: a measure cannot help. Find the evidence: the measure grew while the problem held steady. Ask the what-if question: what would the problem have done without the measure? Pick the answer naming that overlooked possibility. Be wary of answers about population changes (a rate already adjusts for size), about unrelated alternatives the argument never proposes, or about details of who counts what.