Unplugged Practice Test: Logical Reasoning Set 2, Question 8 (Flaw)
Taking the test? Finish it first. This page shows the answer.
- Flaw questions ask you to name the reasoning error the argument commits, not to judge whether its conclusion is true.
- Before you read the choices, describe the argument's error in your own words. Most Flaw questions reward 1 of a small set of recurring patterns, so naming the pattern first keeps the trap answers from sounding right.
- Every question on this test is original, written to the same logical skeletons as real questions, and Steve checked every one himself before release.
The question
Should the proposed limits on streambank grazing take effect, the silt washing into the Marren River each year will fall to a quarter below its present level. Silt in the Marren now forces the regional water authority to spend $8 million a year on extra filtration. The authority would therefore spend roughly $2 million less on filtration each year once the limits were in place.
8. The reasoning above is open to criticism on the grounds that it
- (A)ignores the chance that grazing pushed away from the streambanks would harm other land in ways that raise the water authority's costs elsewhere
- (B)takes for granted that the authority's filtration spending rises and falls roughly in step with the quantity of silt in the river
- (C)offers no grounds at all for expecting that the proposed limits on streambank grazing will in fact be enacted
- (D)rests its case on an appeal to the sympathies of residents who live along the Marren River
- (E)brings up the silt in the Marren mainly to draw attention away from costlier problems facing the water authority
BWhy (B) is correct
The argument takes a quarter drop in silt and turns it straight into a quarter drop off the $8 million filtration bill, or roughly $2 million in savings. That step only works if filtration spending rises and falls in step with how much silt sits in the river. Cutting the first portion of silt might save very little, or a lot more than a flat proportion predicts, and the stimulus never says the relationship is linear. (B) names exactly that unsupported assumption.
Why the other answers are wrong
AWhy (A) is wrong
(A) raises a cost elsewhere, from grazing pushed off the streambanks, which the stimulus never mentions and isn't the gap in the dollar math.
CWhy (C) is wrong
(C) points out the limits might never take effect, but the argument only claims what would happen if they do, so that's not a flaw.
DWhy (D) is wrong
(D) reads sympathy into an argument built entirely on a dollar figure and a percentage.
EWhy (E) is wrong
(E) claims the argument is dodging bigger problems, but it never leaves the topic of silt and filtration costs.
Common questions
What type of question is this?
Flaw. Flaw questions ask you to name the reasoning error the argument commits, not to judge whether its conclusion is true.
How do I get better at Flaw questions?
Before you read the choices, describe the argument's error in your own words. Most Flaw questions reward 1 of a small set of recurring patterns, so naming the pattern first keeps the trap answers from sounding right. The free LSAT cheat sheet lists the wrong-answer patterns to watch for on every type.
