Logical Reasoning · Sufficient Assumption

PrepTest 113 · Section 3 · Question 10

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

Step 1 · Pre-Question Guidance · read this first

Question type

Sufficient Assumption

The one idea it tests

A rule links one quality to another; to reach a conclusion about a group, you need a premise that the group has the first quality.

The basic logic

Map the rule as an arrow: if X, then Y. The conclusion says many members of some group are Y. The magic key says many of that group are X, so the arrow fires.

Tiny words that decide it

“if”
“then”
“so”
“many”

Playground logic

Tommy's patty is a rule: a card that would have been fake is a card he should not have wanted. His top bun says many of the cards he passed up are cards he should not have wanted. The secret sauce clicks the rule into place: many passed-up cards would have been fakes.

Step 2 · The kid story

Tommy's Trading Card Rule

Tommy sat on the porch steps sorting trading cards. "Here is my rule," he told Lulu. "If a card would have turned out to be fake once I traded for it, then I should never have wanted it." Lulu nodded and wrote that down. "So," Tommy went on, "lots of the cards I passed up at the swap meet were cards I should never have wanted." Lulu tapped her pencil. "Your rule only talks about fake cards. How do you know the ones you passed up would have been fakes?" Tommy paused. "Well, the guy selling them had a printer in his backpack." "Then say that," said Lulu. "Many of the cards you skipped would have been fakes. Now your rule can do its job."

Step 3 · Try the model question

If a trading card would have turned out to be fake had Tommy traded for it, then Tommy should not have wanted that card. So many of the cards Tommy passed up are cards he should not have wanted.

Which one of the following, if added as a premise, makes Tommy's conclusion follow?

Step 4 · The courtroom tale

The Case of the Unbuilt Sheds

Judge Ortiz read the town code aloud. "Any shed that would have blocked a neighbor's sunlight is a shed that should not have been planned." Attorney Pell rose. "Therefore, Your Honor, many of the sheds that were never built should not have been planned." Mr. Whitfield cleared his throat. "Counsel has a fine rule and a fine conclusion, but no bridge. Where is the evidence that many of those unbuilt sheds would have blocked anyone's sunlight?" Pell flipped through her notes. "The surveyor's report says most of them were sited on the south fence line." "Then put that in," said the judge. "Now the rule applies, and the conclusion follows."

Step 5 · Work the real question

Open PrepTest 113, Section 3, Question 10 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Turn the rule into an arrow: one condition leads to one result. Note the group named in the conclusion and the result it claims for many of that group. Then hunt for the answer that says many of that same group meet the rule's condition. Check that it uses the arrow forward, not backward, and that it talks about the right group. Test it: add it in, and the conclusion should be forced.