Logical Reasoning · Necessary Assumption

PrepTest 154 · Section 2 · Question 15

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

When evidence is about one group and the conclusion is about another, the argument needs the two groups to match.

The basic logic

The patty describes what one group believes. The secret sauce links that belief to a rule about a different, larger group, and the top bun predicts what the larger group will do. The required assumption is that the first group's attitude is not very different from the larger group's. Pull the plank: if they differ a lot, the prediction falls.

Tiny words that decide it

“clearly”
“tends to”
“likely”
“since”

Playground logic

Owen sees the kids in the art club buying lots of glitter and says they think the craft fair will be a hit. Since kids who expect a hit usually vote to keep the fair, Owen predicts the whole school will vote to keep it. The secret sauce: the art club kids think like the rest of the school. If they don't, his top bun has no support.

Step 2 · The kid story

Glitter and the Big Vote

Owen burst into the classroom. "The art club kids just bought ALL the glitter at the store. They must think the craft fair will be amazing. And when kids think the fair will be amazing, they vote to keep it. So the whole school will vote to keep the craft fair!" Priya sipped her juice box. "Owen, how many kids are in art club?" "Twelve." "And how many kids vote?" "Four hundred." Priya nodded. "So you're guessing what four hundred kids think from what twelve art kids think. That only works if the art kids feel about the fair the way everyone else does. Art kids might love craft fairs more than anyone." Owen looked at his glitter-covered hands. "Okay, that's fair."

Step 3 · Try the model question

Owen: The art club members are buying lots of glitter, so they clearly expect the craft fair to be a big success. Since students who expect the fair to succeed tend to vote to keep it, the school will probably vote to keep the craft fair.

Owen's argument requires which one of the following assumptions?

Step 4 · The courtroom tale

The Case of the Cookie Forecast

Mayor Dill's lawyer, Ms. Quill, told the court: "Bakers in town are ordering huge amounts of flour. Clearly, they expect a booming season. Townsfolk who expect a booming season tend to support the mayor. So the mayor will win." Mr. Whitfield rose. "Your Honor, bakers are a few dozen people. Voters are thousands. Counsel needs the bakers' mood to roughly match everyone else's." Ms. Quill paused. "Well, bakers are pretty normal." "Then you need to say so," said Mr. Whitfield, "because if bakers are unusually cheerful, your forecast has no legs." The judge wrote: assumption noted.

Step 5 · Work the real question

Open PrepTest 154, Section 2, Question 15 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Map the burger: which group the evidence describes and which group the conclusion is about. If they differ, the needed assumption bridges them. Check each answer with the Pull the Plank test: negate it and see if the argument falls. The right answer is often modest, like 'not very different'. Cross out answers that add new causes, extra loyalties, or claims stronger than the argument needs.