Logical Reasoning · Inference (Must Be True)

PrepTest 129 · Section 2 · Question 25

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

Step 1 · Pre-Question Guidance · read this first

Question type

Inference (Must Be True)

The one idea it tests

A rule only kicks in for one group, so if nobody from that group showed up, the rule never applied.

The basic logic

Diagram the rule: big item AND from outsider who was never an insider means it must be logged. Then the facts: everything came from insiders or past insiders. So the trigger never happened. What must be true is only that nothing needed logging, not that nothing was logged.

Tiny words that decide it

“all”
“who are not”
“only”
“must”

Playground logic

The library rule only reaches long books borrowed by kids who are neither fourth graders nor past fourth graders. Tommy's book club borrowed only from fourth graders and past fourth graders, so the rule never kicked in: none of their books had to be signed in. That is all the lunchbox holds.

Step 2 · The kid story

The Red Notebook Rule

The school library has a red notebook rule: any book longer than 300 pages, borrowed by someone who is not a fourth grader and never was one, must be signed into the red notebook. Tommy started a book club. Lulu checked every member with her clipboard. 'Everyone is a fourth grader or a past fourth grader,' she said. 'So none of our books ever had to go in the red notebook.' Marcus jumped up. 'So the club wrote nothing in it, ever!' Lulu shook her head. 'We don't know that. Maybe Tommy signed something in just for fun. The rule just never required it.' Tommy coughed. 'I may have signed in a comic book. It looked official.' Mr. Whiskers sat on the red notebook, ending the discussion.

Step 3 · Try the model question

Library rule: every book over 300 pages borrowed by someone who is neither a fourth grader nor a former fourth grader must be signed into the red notebook. Tommy's book club borrowed books only through fourth graders and former fourth graders.

If everything above is true, which choice must also be true?

Step 4 · The courtroom tale

The Pool Pass Rule

The town pool has a rule: any guest staying over an hour who is not a town resident and never was one must sign the guest log. The pool manager was charged with breaking it at a birthday party. Mr. Whitfield waved a list. 'Your Honor, every guest at that party was a resident or a former resident. The rule never reached a single one of them.' The prosecutor objected. 'Then the manager signed no one into the log!' 'That does not follow,' said Mr. Whitfield. 'He may have signed people in anyway. What must be true is only that nobody had to be signed in.' The judge agreed and dismissed the charge.

Step 5 · Work the real question

Open PrepTest 129, Section 2, Question 25 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Diagram the rule carefully, including every condition stacked on its trigger. Then check the facts against the trigger: does any case meet all its conditions? If none can, the only sure thing is that the rule required nothing. Read each choice and ask, could this be false while the facts stay true? Watch for choices that say nothing was done at all, or that drop one of the stacked conditions.