Logical Reasoning · Necessary Assumption

PrepTest 113 · Section 3 · Question 14

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

Step 1 · Pre-Question Guidance · read this first

Question type

Necessary Assumption

The one idea it tests

Ruling out one copying route does not prove independence; the argument must also assume there was no other shared route.

The basic logic

Evidence knocks out one way that one person could have learned from the other. The conclusion says both came up with the idea on their own. Pull the plank: if both had learned it from a common outside source, the conclusion collapses.

Tiny words that decide it

“thus”
“each”
“on its own”

Playground logic

Priya's patty: Frankie had sketches first, and his bus doodle taught Sharon nothing. Her top bun: each kid invented the frog alone. The secret sauce is that neither one got the frog from some third place, like the same library book.

Step 2 · The kid story

The Two Paper Frogs

Sharon brought a brand new paper frog to show-and-tell. It jumped higher than any frog in history. The next day Frankie stomped in. "I invented that frog first!" He showed folding sketches dated three weeks earlier. He also said he had doodled the frog for Sharon on the bus. But Lulu looked at the doodle. It was just a green blob with eyes. "That doodle could not teach anyone to fold anything," she said. Priya sipped her juice box. "So Frankie had it first, and Sharon did not learn it from him. I declare that each of you invented the frog on your own." Lulu frowned. "Wait. What if both of you found it in the same library book?" Frankie and Sharon looked at each other. Uh oh.

Step 3 · Try the model question

Sharon showed her paper frog first, but Frankie had sketches of it from weeks earlier. Frankie says he showed Sharon on the bus, yet his bus doodle revealed nothing useful about folding. So Frankie and Sharon each invented the frog on their own.

The conclusion depends on which assumption?

Step 4 · The courtroom tale

The Case of the Twin Melodies

Two songwriters wrote nearly the same tune. Mr. Whitfield represented the second one. "Your Honor, my client recorded the tune first. The other writer's demo tape came earlier, yes, but the only message he sent my client was a voicemail with three random notes. Nothing useful. So each of them created this tune independently." Judge Amara peered over her glasses. "Mr. Whitfield, you have shown neither one learned it from the other. But both attended the same summer music camp, where the teacher hummed tunes all day. Did either of them borrow it from her?" Mr. Whitfield tugged his suspenders. "I will need to assume not, Your Honor."

Step 5 · Work the real question

Open PrepTest 113, Section 3, Question 14 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the conclusion that two parties arrived at something on their own. Then list what the evidence rules out: usually one party learning from the other. Ask what other path could make the conclusion false, such as a shared outside source. Look for the answer that closes that path, and test it by negating it: if the negation breaks the conclusion, you have the needed assumption.