Logical Reasoning · Must Be True

PrepTest 103 · Section 2 · Question 6

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

Combine the facts: when two cases share everything except one factor, and that factor is blamed for failure in one case, the successful case must differ on that factor.

The basic logic

Line up what the two cases share and the one difference the passage blames. If one case succeeds and the other fails only because of that difference, the successful case must have more of it, at least where it matters. Stay inside the lunchbox; ignore averages and what-ifs.

Tiny words that decide it

“even if”
“too”
“at least”

Playground logic

Nadia's sand castle stands tall on the thick sandbox board, while the same castle would sink through Owen's thin cardboard because cardboard of the same stuff is too thin. Put the patties together and the conclusion pops out: at least where Nadia's castle sits, her board is thicker than Owen's cardboard.

Step 2 · The kid story

The Tower That Would Sink

Nadia built a giant block tower on her craft table. It was taller than Kevin. Owen wanted the same tower on his own table. 'It won't work,' said Nadia. 'Our tables are made of the same wood, but yours is way too thin. It would bend and the tower would sink right through.' Owen pouted. 'Even if my table were as level as yours?' 'Even then. Too thin.' Lulu wrote on her clipboard. 'Same wood. Hers holds the tower. His would bend because it is too thin. So her table must be thicker than his, at least where the tower stands.' Marcus said, 'So her tower is taller than all towers ever!' Priya shook her head. 'Nobody said that, Marcus. Stay in the lunchbox.'

Step 3 · Try the model question

Nadia's huge block tower stands on her craft table. Owen's table is made of the same wood, but even if the tables were equally level, the same tower could not stand on it, since Owen's table is so thin it would bend under even a small part of that weight.

Given these facts, which choice is guaranteed to be true?

Step 4 · The courtroom tale

The Bridge Planks

A carpenter sued a town after his heavy statue was refused for the east bridge. Mr. Whitfield, for the town, called an engineer. 'The west bridge holds the same statue right now. Both bridges use identical planks from the same mill. Even with the same load spread evenly, the east bridge planks are too thin to hold even part of that weight.' The judge leaned forward. 'Then I can conclude one thing for certain: the west bridge planks are thicker than the east bridge planks, at least where the statue sits.' The carpenter's lawyer asked, 'Is the west bridge stronger everywhere?' The judge smiled. 'I did not say that. Only what the facts force.'

Step 5 · Work the real question

Open PrepTest 103, Section 2, Question 6 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

List each fact in plain abstract terms: what is the same between the two cases, and what single difference is blamed for the failure? Then ask what must follow when you combine them. Use the Spoiler Test: try to imagine each choice false while every fact stays true. Eliminate choices about averages, extra causes, or 'what would happen if' claims the facts do not force. Keep the modest one with a hedge like 'at least'.