Logical Reasoning · Necessary Assumption

PrepTest 155 · Section 2 · Question 7

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 an argument uses what got caught somewhere to judge what a group was doing, it needs the catching to happen mostly during that activity.

The basic logic

Patty: a place that collects samples has none from one subgroup. Top bun: that subgroup probably skipped a certain activity. The gap: the samples are linked to that activity. If the catching happened mostly at some other time, the missing subgroup tells us nothing about the activity.

Tiny words that decide it

“none”
“probably”
“therefore”

Playground logic

Nadia sees that no kindergarten sneakers are stuck in the mud by the soccer goal and decides kindergartners don't play soccer with big kids. Her secret sauce: sneakers get stuck there mostly during soccer games. Pull that plank and her conclusion sinks.

Step 2 · The kid story

The Mystery of the Muddy Sneakers

Behind the soccer goal at school sits a deep mud puddle. Over the year, dozens of sneakers have been pulled out of it, left behind by kids who got stuck. Nadia sorts them all. "Fifth grade, fourth grade, third grade... but not a single kindergarten sneaker!" she announces. "So kindergartners probably don't play soccer with the big kids." Priya sips her juice box. "That only works if sneakers get stuck in that mud mostly while kids are playing soccer. If kids get stuck while chasing a ball that rolled over during lunch, or while taking a shortcut, the missing sneakers don't tell you about soccer at all." Nadia frowns at the pile. Mr. Whiskers sits in one sneaker and refuses to leave.

Step 3 · Try the model question

Many sneakers have been pulled out of the mud puddle beside the soccer goal, lost by kids who got stuck there. None of the sneakers belong to kindergartners. So kindergartners probably do not play soccer with the older kids.

Nadia's argument depends on which one of these assumptions?

Step 4 · The courtroom tale

The Case of the Missing Tiny Kites

A park ranger testified that hundreds of kites had been tangled in a tall oak tree over many years. "Not one was a small child's kite," she said. "So small children probably do not fly kites in this park." Mr. Whitfield stood up. "Your Honor, this reasoning works only if kites get tangled in that oak mostly while being flown. Suppose most of those kites were tossed up there by pranksters or blown in from a nearby shop. Then the missing small kites tell us nothing about who flies kites here." The ranger hesitated. "I assumed they got stuck while flying." The judge nodded. "That assumption carries your whole conclusion, Ranger."

Step 5 · Work the real question

Open PrepTest 155, Section 2, Question 7 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

State the conclusion about what a subgroup did and the evidence about what was found in some place. Ask how the place connects to the activity. The needed assumption links being caught there to doing that activity. Negate each choice: if the negation breaks that link, you have the plank. Skip choices about the subgroup's abilities, comparisons with other groups, or how popular the location was.