Logical Reasoning · Must Be True

PrepTest 142 · Section 2 · Question 3

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

Step 1 · Pre-Question Guidance · read this first

Question type

Must Be True

The one idea it tests

If two things together guarantee a result, and you already know one of them is present, then the other one alone would now guarantee it.

The basic logic

Map the conditional: A and B together lead to C. Find which half the passage confirms. The safe inference is a new conditional: if the missing half shows up, the result follows. Ignore distracting extra claims that only say something 'may' happen.

Tiny words that decide it

“if”
“both”
“must”
“may”

Playground logic

Lulu's pet show rule: groomed AND well-behaved means a ribbon. Biscuit is already well-behaved. So the only thing in the lunchbox is: if Biscuit is groomed, he gets a ribbon. Nothing says he IS groomed.

Step 2 · The kid story

Biscuit at the Pet Show

Lulu was the judge of the backyard pet show, and she wrote the rule on her clipboard: "Any pet that is both well-groomed and well-behaved gets a blue ribbon." Tommy added, "Even a pet that hasn't learned tricks could win." Then Biscuit trotted up. He sat. He stayed. He did not chew a single shoelace. "Well-behaved," Lulu said, checking a box. Marcus shouted, "So Biscuit wins!" Lulu shook her head. "I only know half. Nobody has brushed him yet. He has burrs in his tail." Priya smiled. "Here's what we really know: if Biscuit gets brushed, he gets a ribbon." Marcus sighed and went to find a brush. Biscuit wagged, unaware of the logic.

Step 3 · Try the model question

Pet show rule: any pet that is both well-groomed and well-behaved gets a blue ribbon. A pet without tricks may still win a ribbon if it is well-behaved. Biscuit knows no tricks, but he is well-behaved.

If everything above is true, which one of the following has to be true as well?

Step 4 · The courtroom tale

The Two-Key Trophy

At the county fair hearing, the clerk read the rule: "A pie wins the gold trophy if it is both baked on time and approved by the taster." The baker's lawyer jumped up. "The taster approved my client's pie! So she won!" The judge raised an eyebrow. "Was it baked on time?" Silence. "Counselor," said the judge, "you have one key in a two-key lock. What I can say for certain is this: if the pie was on time, it wins. Bring me the oven log." Mr. Whitfield, sitting in the back, whispered to his neighbor, "Half a recipe is not a pie."

Step 5 · Work the real question

Open PrepTest 142, Section 2, Question 3 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the main conditional as two conditions joined by and, leading to a result. Note which condition the passage says is met now, and which is unknown. Treat softer statements about what may happen as weaker than the rule. The answer that must be true usually says: if the unknown condition holds, then the result follows. Reject choices that claim the unknown condition is true, flip the arrow, or add a condition the rule never mentions.