Logical Reasoning · Flaw

PrepTest 120 · Section 4 · 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

Flaw

The one idea it tests

Ruling out one of two listed options proves the other only if those two are the only options.

The basic logic

The argument names a goal, lists two ways to reach it, rejects one, and declares the other must happen. Look for the hidden claim that the list was complete. That is Only Two Doors: other doors may exist.

Tiny words that decide it

“could”
“also”
“since”
“must”

Playground logic

Tommy says the lemonade stand must cut costs, it could buy fewer lemons or use less sugar, he refuses to buy fewer lemons, so they must use less sugar. The patty lists two doors; the top bun picks the one left. The flaw is Only Two Doors: cheaper cups, borrowed pitchers, or other savings were never ruled out.

Step 2 · The kid story

Only Two Doors at the Lemonade Stand

The lemonade stand had a money problem. Tommy tapped the cash box. "We spent too much last week. We need to cut costs. We could buy fewer lemons. Or we could use less sugar. I am not buying fewer lemons, because lemons are the whole point. So we must use less sugar." Kevin gasped. "Sour lemonade? Nobody will buy it!" Priya calmly sipped her juice box. "Tommy, who said those are the only two ways?" She pointed. "We bought fancy striped cups. We could use plain ones. We paid for ice, but Grandpa has an ice maker. And Marcus ate three cookies from the snack jar without paying." Marcus looked at the ceiling. Lulu wrote on her clipboard: 'Two doors named does not mean two doors exist.'

Step 3 · Try the model question

Our lemonade stand must spend less next week. We could buy fewer lemons. We could also use less sugar. Since we will not buy fewer lemons, we have to use less sugar.

What is the mistake in Tommy's reasoning?

Step 4 · The courtroom tale

The Case of the Shrinking Zoo Budget

A city zoo had to trim its spending. The zoo director testified, "We could close the reptile house, or we could shorten the opening hours. We refuse to close the reptile house. Therefore we must shorten the hours." Visitors who loved early mornings sued. Mr. Whitfield rose. "Your Honor, the director speaks as if only two doors exist. What about the gift shop that loses money? The unused parking lot? The fountain that runs all night?" The director blinked. "Those were not on my list." Judge Okafor smiled. "Your list was short, not complete. Rejecting one choice does not force the other."

Step 5 · Work the real question

Open PrepTest 120, Section 4, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the goal, then the options the argument lists for reaching it. Notice which option it rejects and which it declares must happen. Ask whether the argument ever shows that its list covers every possibility. The right answer names the unproven belief that only the listed options can reach the goal. Cross off answers that point at something the argument never actually relies on.