Logical Reasoning · Sufficient Assumption

PrepTest 139 · Section 4 · Question 21

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

Build a bridge. The conclusion starts from a condition the premises never mention; you need a key that connects that new condition to the start of the premise chain.

The basic logic

Map the chain: first event leads to a second, which leads to the outcome. Then read the conclusion's 'if' part. If it is a new idea, the magic key links that new idea to the first event of the chain. Do the Click Test: add the answer and see if the conclusion locks shut.

Tiny words that decide it

“if”
“will”
“certainly”
“thus”

Playground logic

Tommy's premise chain: if Biscuit learns to roll over by Saturday, Grandpa signs him up, and signed-up dogs march. His top bun: if Biscuit is as smart and patient as Tommy says, he will march. The magic key connects 'smart' to 'learns by Saturday', and then the whole chain clicks.

Step 2 · The kid story

Biscuit's Big Parade

The neighborhood pet parade is Sunday. Grandpa makes a promise: "If Biscuit learns to roll over by Saturday, I will sign him up." And the parade rules say every signed-up dog gets to march. Tommy beams. "So if Biscuit is as smart and as patient as I keep telling everyone, he will march!" Priya taps her juice box. "Hold on. Your chain starts with rolling over by Saturday. Your conclusion starts with being smart and patient. Those are different starting lines." Lulu draws arrows on her clipboard. "We need one more arrow. Smart leads to rolling over by Saturday." Tommy thinks. "Okay. If Biscuit is as smart as I say, he will learn the trick by Saturday." Lulu adds the arrow. Click. Biscuit wags.

Step 3 · Try the model question

Grandpa promises that if Biscuit learns to roll over by Saturday, he will sign Biscuit up for the pet parade, and every signed-up dog marches. So if Biscuit is as smart and as patient as Tommy claims, Biscuit will march in the parade.

Which assumption, once added, makes Tommy's conclusion follow for sure?

Step 4 · The courtroom tale

The Bakery Promise

Mr. Whitfield faced the jury. "The mayor promised: if the new oven is installed by June, she will approve the bakery's permit. And every approved bakery may open on Main Street." He paused. "My opponent concludes that if the bakery's recipes are as good as the owner claims, it will open on Main Street." The judge frowned. "The promise is about an oven, not recipes." "Exactly, Your Honor. That conclusion only follows if good recipes guarantee the oven is in by June. Without that bridge, the chain never starts." The opponent scrambled for a witness who could connect recipes to ovens. Nobody could.

Step 5 · Work the real question

Open PrepTest 139, Section 4, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the premises as arrows: first condition, then the promised action, then the final result. Write the conclusion as its own arrow. Find the piece of the conclusion's 'if' part that never appears in the premises. The right answer builds a bridge from that piece to the very first link of the chain. Run the Click Test: add the answer and see whether the conclusion now must follow. Arrows pointing the wrong way, or answers about a different link, fail.