Unplugged Practice Test: Logical Reasoning Set 3, Question 10 (Necessary Assumption)
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- Necessary Assumption questions ask for the unstated premise the argument depends on.
- Try the negation test. If negating a choice wrecks the argument, that choice is the necessary assumption.
- 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
For decades the Halden lathe has been thought incapable of cutting helical threads. Using a newly devised tool-mark assay, machinists dated a bracket recovered from a decommissioned production line to within 6 weeks of that line's opening, a period when the Halden was the only lathe on that floor, so the bracket was almost certainly finished on the Halden. That long-standing judgment about the Halden is therefore probably mistaken.
10. The argument depends on which one of the following assumptions?
- (A)No lathe other than the Halden stood anywhere in the plant during the month that production line opened.
- (B)Any machine incapable of finishing a bracket of the recovered kind is also incapable of cutting helical threads.
- (C)Finishing a bracket of the kind recovered from that line shows a capacity to cut helical threads.
- (D)The Halden was certainly fitted with a carriage able to hold stock of the dimensions the bracket required.
- (E)The bracket is the earliest sign that the Halden could cut helical threads.
CWhy (C) is correct
The argument concludes the old judgment that the Halden lathe couldn't cut helical threads is probably mistaken, because a bracket dated to when the Halden was the only lathe on that floor was almost certainly finished on it. For that bracket to overturn the old judgment, finishing a bracket like it has to actually show the capacity to cut helical threads. Without that link, a Halden-finished bracket would prove nothing about helical threads, so the argument needs (C).
Why the other answers are wrong
AWhy (A) is wrong
(A) goes further than the argument requires, since the premise already limits itself to the Halden being the only lathe on that floor, not the only one anywhere in the plant.
BWhy (B) is wrong
(B) reverses the needed link, since the argument needs finishing the bracket to show the capacity, not the other way around.
DWhy (D) is wrong
(D) isn't required. The conclusion is about what the bracket shows about capability, not about the Halden's physical fittings.
EWhy (E) is wrong
(E) doesn't matter, since being the earliest sign or not has no bearing on whether the bracket shows the Halden could cut helical threads.
Common questions
What type of question is this?
Necessary Assumption. Necessary Assumption questions ask for the unstated premise the argument depends on.
How do I get better at Necessary Assumption questions?
Try the negation test. If negating a choice wrecks the argument, that choice is the necessary assumption. The free LSAT cheat sheet lists the wrong-answer patterns to watch for on every type.
