Logical Reasoning · Weaken

PrepTest 148 · Section 3 · Question 2

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 by comparison: something bad happened to an old case, so the same will happen to a new case that shares one feature. Pop it by showing the new case lacks the thing that caused the harm.

The basic logic

The patty: in the old case, a practice caused a buildup that eventually ruined things. The top bun: the new cases that use the same practice will probably suffer the same fate. The secret sauce: the new cases do the practice in the same harmful way. Weaken by showing the new cases do it in a way that avoids the harmful part.

Tiny words that decide it

“similar”
“thus”
“likely”
“continue”

Playground logic

Grandpa's old treehouse rotted because rain soaked its boards year after year, so Marcus predicts the new treehouse will rot too. The secret sauce is that the new boards soak up water the same way. Show the new boards are sealed, and the argument pops.

Step 2 · The kid story

The Soggy Treehouse Prophecy

Grandpa's old treehouse stood for thirty summers. But every rainy season, water soaked into the boards and stayed there. Little by little, the wood went soft. One spring, the floor gave way, and the whole thing came down with a sad crunch. Now Tommy and Owen were building a new treehouse in the same oak. Marcus folded his arms. "Doomed. Rain rotted the old one. Rain will rot this one too." Lulu flipped her clipboard. "Did you check the boards?" Owen grinned and knocked on a plank. "Treated wood, plus three coats of sealer. Water beads right off." Priya sipped her juice box. "Same rain, different boards. The old treehouse rotted because water got in. If water can't get in, the story changes." Marcus quietly put away his doom sign.

Step 3 · Try the model question

Grandpa's old treehouse lasted thirty years. But rain kept soaking into its boards, and eventually the wood rotted and the treehouse fell. So, Marcus argues, the new treehouse being built in the same tree will probably rot and fall as well.

Which of these, if true, does the most damage to Marcus's argument?

Step 4 · The courtroom tale

The Case of the Rusty Bridge

A town council argued that its new footbridge must be torn down. "The old iron bridge rusted through after fifty years," said their lawyer, "because rain sat in its joints. The new bridge will rust too." Mr. Whitfield stood and adjusted his suspenders. "Your Honor, the new bridge is made of coated steel, with drainage holes at every joint. Rain can't sit there." The judge peered over her glasses. "So the old bridge failed because water pooled in the joints, and the new one is built so water doesn't pool?" "Exactly. Same weather, different design." The judge closed her folder. "The comparison only works if the new bridge shares the weakness. It doesn't. Motion denied."

Step 5 · Work the real question

Open PrepTest 148, Section 3, Question 2 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Map the comparison: an old case suffered harm because a practice caused a slow buildup, and the conclusion predicts the same for new cases that use the practice. The hidden assumption is that the new cases use the practice in the same harmful way. Look for the answer that breaks the comparison by showing the new cases avoid the harmful part. For each choice, ask: does this make the new cases less like the old one in the way that matters?