LR Question 21 Explanation
QUESTION TEXT: Consumer magazine: Because front-loading washers use less …
QUESTION TYPE: Necessary Assumption
CONCLUSION: Front-loading machines require a special detergent in order to properly clean clothes.
REASONING: Ordinary powder detergent doesn’t fully dissolve in front loading machines.
ANALYSIS: Pay close attention anytime terms switch, especially when it seems “reasonable” to assume that they mean the same thing.
The evidence is that “powder won’t dissolve fully”. The conclusion is that “clothes won’t get fully clean”. That sounds reasonable, but who says powder needs to dissolve fully in order to clean clothes? That’s just an assumption the argument is making.
___________
- Negate this: “One top loading machine in Mongolia uses half an ounce more water than other top loading machines”. That certainly doesn’t wreck the argument.
- Negate this: “A detergent designed for front loading washers also dissolves well in top loading washers”. That just shows that the detergent can work in both types. Great!
- This answer refers to all washing machines. So you could negate it by saying “Front loading machines require special detergent, but top loading machines can use any kind”. The argument is only about front loading machines.
- CORRECT. I don’t think this answer is properly formulated. I think it should have said “An ordinary powder detergent does not get clothes really clean unless it dissolves readily”. You could negate the answer as written by saying that liquid detergent doesn’t need to dissolve readily, but powder detergents do. That wouldn’t wreck the argument. That said, this is the best answer.
Negation: A detergent can get clothes really clean even if it doesn’t dissolve readily. - We know that top loading washers use more water, and they may get clothes cleaner with ordinary detergent. But that doesn’t mean that more water is always good. Maybe there’s a washer that uses even more water but doesn’t work well.
Free Logical Reasoning lesson
Get a free sample of the Logical Reasoning Mastery Seminar. Learn tips for solving LR questions
Hi, I'm Graeme Blake
I run LSAT Hacks, and got a 177 on the LSAT. The single best thing I've ever made is the set of LSAT Mastery seminars. They show you how to think like a 170+ scorer when doing questions. Get them here: Mastery Seminars
I guarantee you'll like them, or you get your money back within 7 days. There's no risk. Check the reviews, people have said they improved within a few days.
---------
Photos and Updates: You can follow me on Instagram here
For updates, sign up for my email list. I update whenever I have new posts.
I’m not sure why Answer D is better than Answer B since the question asks for a Necessary Assumption.
I negated Answer B differently (Detergent formulated for FLW does NOT dissolve more readily) which wrecked the conclusion by removing the salient difference that the argument relies upon.
I agree Answer D is weird because it answers with a Sufficient Assumption to a NA question. Accordingly, negating Answer D (It’s not true that [detergent gets clothes really clean –> detergent dissolves readily in washer]) does not wreck the conclusion. Perhaps a detergent formulated for FLW simply requires less water to be just as effective. Or perhaps a detergent formulated for FLW has a special cleaning agent to compensate for not dissolving as readily.
In addition to the above reasons, I initially POE’d Answer D as a trap answer because it provides a “firmer” (conditional guarantee!) response more appropriate to a SA question. In contrast, Answer B provides a “softer” (comparative) response appropriate to most NA questions.
Actually, I now see why Answer B is incorrect. For Answer B to be correct, the specially formulated detergent’s dissolving should have been compared to ordinary detergent rather than its performance in different washers – very sneaky!
I completely agree that Answer D is not properly formulated. This question should probably be removed.
Actually (again), while I initially viewed Answer D as a SA, after closer scrutiny I think otherwise:
P: O –> not DR
C: RC –> not O –> S
O (Ordinary detergent in front-loader)
DR (Dissolves Readily in front-loader)
RC (Really Clean in front-loader)
S (Special detergent in front-loader)
The conclusion introduces two new elements (RC and S) but only RC is linked to the premise by [Answer D]: [RC –> DR] –> not O
S remains a gap unsupported by any premise: [not O–> S]
This means another NA could be (for example), “There is no additive to ordinary detergent that would cause it to dissolve readily in a front-loader.”
Because Answer D, despite its SAish format, does not completely bridge the gap between the premise and conclusion, it’s not an actual SA but rather (correctly) a NA.
Well played, LSAC, well played.
I don’t think I thought D was a SA.
But I don’t think it’s necessary either. Because this is technically a negation that doesn’t wreck the argument: “Liquid detergent doesn’t need to dissolve readily, but powder detergents do”
That is inconsistent with D, but consistent with the argument, so I don’t think it passes the NA test?
Yes!! Exactly! Thank you for writing precisely the thing I was going to write. I was about to choose D but then caught myself saying “wait… the conclusion sentence contains neither of the premises! It’s just asserting a conclusion. Ergo, the assumption being made IS that conclusion.” How is circular reasoning NOT a necessary assumption for an unsupported assertion? That’s what it by definition is: an argument which assumes it’s own conclusion. Seems “necessary” to me since there are no premises anywhere to support it.
Thank you for waxing eloquent on this Modus Tollens vs Non Sequitur distinction several years ahead of my time, friend!
“One top loading machine in Mongolia” Hahaha I can’t with you. I’ve been laughing all the way through your examples (which are excellent by the way)
Quick comment on (C):
Even if it said “front-loading machines” instead of washing machines, it would still be wrong.
It would basically just restate the conclusion of the argument, which of course is not necessary for the argument to be right.
It would be a sufficient assumption though I think, but would make no sense because it would deem the whole first sentence of the stimulus unnecessary.
Hmm, I think you’re right. It’s an unusually circular answer.