Logical Reasoning · Weaken

PrepTest 113 · Section 3 · Question 19

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

Step 1 · Pre-Question Guidance · read this first

Question type

Weaken

The one idea it tests

An argument that eliminates every innocent explanation by using if-then premises can be popped by showing one of those if-thens might not hold.

The basic logic

The argument says: if case one, we would see X; if case two, we would see Y; we see neither, so something went wrong. Attack a hidden plank: the idea that knowing what someone likes means you will give them what they like. Show the giver might have another motive.

Tiny words that decide it

“if”
“would have”
“must”
“either”

Playground logic

Tommy reasons that a close friend would have picked chocolate and a stranger would have left a name tag, so the vanilla cupcake with no tag must be a cafeteria mix-up. The secret sauce is that anyone who knows his favorite would want to please him with it. If some kids give treats for other reasons, like a tease, the whole burger tips over.

Step 2 · The kid story

The Vanilla Cupcake Mystery

On Tuesday a cupcake appeared on Tommy's desk. It was vanilla. Tommy hates vanilla. 'Detective time,' said Marcus. Tommy reasoned it out. 'If a close friend left this, they would know I love chocolate. So it would be chocolate. If someone who barely knows me left it, they would have stuck a name tag on it. But there is no tag. So the cafeteria must have messed up. Either it was supposed to be chocolate, or it needed a tag, or it was meant for somebody else.' Priya sipped her juice box. 'Tommy, Nadia knows you super well. And last week she gave Owen a cookie he hates, just to tease him for being picky.' Tommy stared at the cupcake. Across the room, Nadia was grinning.

Step 3 · Try the model question

If a close friend left the cupcake, it would be chocolate, since a close friend knows Tommy's favorite. If someone who barely knows Tommy left it, it would have a name tag. The cupcake is vanilla and has no tag. So the cafeteria must have made a mistake.

Suppose one of these is true. Which would do the most damage to Tommy's reasoning?

Step 4 · The courtroom tale

The Case of the Plain Gray Scarf

A shop owner was accused of mixing up a knitted gift. The prosecutor said, 'If the gift came from a close friend, it would be red, the receiver's favorite. If it came from a stranger, it would include a card. It is gray, with no card. So the shop blundered!' Mr. Whitfield stood, thumbs in his suspenders. 'Your Honor, the receiver's best friend testified she chose gray on purpose. It was a running joke between them, not an attempt to please.' The prosecutor's mouth opened, then closed. Judge Amara Cole said, 'So a close friend can know your favorite and still choose otherwise. The shop is cleared.'

Step 5 · Work the real question

Open PrepTest 113, Section 3, Question 19 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Lay out the argument's if-then premises and how the observed facts knock out each innocent option. Then look at each if-then and ask what it quietly assumes about why people act. Find the answer showing that one of those if-thens could fail, so an innocent explanation survives. Avoid answers that only make the other option rarer, or that just add a general fact about the helper's record.