Logical Reasoning · Sufficient Assumption

PrepTest 123 · Section 2 · Question 13

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

When an argument converts 'half of the material came from there' into a count, it needs nothing to have gone missing on the way.

The basic logic

Do the math the argument does. All of group 1 went into group 2 and made up half of group 2's stuff, and every item holds the same amount. That gives group 2 twice the items only if every bit of group 1's stuff made it over. Find the magic key that guarantees no loss.

Tiny words that decide it

“all”
“since”
“so”
“exactly”

Playground logic

Lulu's patty: every old necklace was taken apart for bracelets, and those beads are half the beads in the bracelets. Her top bun: there are twice as many bracelets as necklaces. The magic key clicks once every single necklace bead made it into a bracelet, none lost under the couch.

Step 2 · The kid story

The Great Bead Count

Lulu takes apart all of her old necklaces to make bracelets for the class. Each necklace has exactly twenty beads, and so does each bracelet. When she is done, she announces, 'Half the beads in my bracelets came from the old necklaces. And every necklace got used. So I made twice as many bracelets as I had necklaces.' Tommy says, 'Math checks out.' Priya sips her juice box. 'Only if every bead from the necklaces actually ended up in a bracelet.' Just then, Biscuit trots by with three beads stuck to his nose, and Mr. Whiskers bats one under the couch. Lulu groans. 'If some beads got lost, the necklace beads are less than I counted, and my math is off.' She writes the magic key on her clipboard: no bead left behind.

Step 3 · Try the model question

Every old necklace and every new bracelet has exactly twenty beads. All of the old necklaces were taken apart for the bracelets. Half the beads in the bracelets came from those necklaces. So there are twice as many bracelets as there were necklaces.

Which choice, if assumed, would make Lulu's conclusion follow for sure?

Step 4 · The courtroom tale

The Case of the Puzzle Pieces

A toy maker was accused of shorting a store. The store said, 'Every old puzzle was broken up to make new puzzles, old pieces were half the pieces in the new ones, and all puzzles have the same piece count. So there must be twice as many new puzzles as old!' Mr. Whitfield raised a finger. 'Your Honor, that holds only if every old piece made it into a new puzzle. Our factory floor sweeper found a whole bucket of pieces.' The judge nodded. 'Without that key, the count does not follow.' The store's lawyer looked at the bucket and frowned.

Step 5 · Work the real question

Open PrepTest 123, Section 2, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write out the numbers the argument leans on: what was transferred, what share it made up, and what stays the same per unit. Ask what has to be true for the count to come out exactly as claimed. Usually it is that nothing was lost or added during the move. Test each answer with the Click Test: add it and see if the conclusion is now guaranteed. Ignore answers about quality, future use, or comparisons to other things.