PT70.S1.Q21 - consumer magazine: because front-loading washers

hlsat180hlsat180 Free Trial Member
edited June 2016 in Logical Reasoning 362 karma
This is a Necessary Assumption question. Could someone explain how Answer B is incorrect and Answer D is correct?

Answer B speaks to the gap (Detergent formulated for front-load dissolves more readily). Negating Answer B (Detergent formulated for front-load does NOT dissolve more readily) wrecks the conclusion because it removes the salient difference compared to ordinary detergent that the argument makes.

In contrast, Answer D provides a Sufficient Assumption to a Necessary Assumption question. 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 as an ordinary detergent in a TLW. Or perhaps a detergent formulated for FLW has a special cleaning agent to compensate for not dissolving as readily. So on and so forth: In all cases, Sufficient does not equal Necessary.

Furthermore, working from wrong-to-right I eliminated Answer D as a trap. Answer D 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.

Thanks in advance for your help!
https://7sage.com/lsat_explanations/lsat-70-section-1-question-21/

Comments

  • nye8870nye8870 Alum ⭐
    edited June 2016 1749 karma
    https://7sage.com/lsat_explanations/lsat-70-section-1-question-21/

    ac-B doesn't have to be true because the detergent specially made for front loaders could very well dissolve more readily in a top-loader. In fact that would would make plenty good sense as we are told that there is more water in the top-loader and so it is conceivable that it would dilute even easier. ac-D says that if a detergent does not dissolve readily in a washer, then that detergent does not get clothes really clean. This MUST be true for the conclusion to hold. (the negation of this statement says: if a detergent does not dissolve readily in a washer, it IS still possible that clothes could get really clean.) This contradicts the conclusion.
  • hlsat180hlsat180 Free Trial Member
    362 karma
    Thanks for the explanation! I see and agree with your reasoning why B doesn't have to be true: 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!

    However, negating Answer D (a Sufficient Assumption) only rules out one way to close the gap in the argument, rather than ruling out the conclusion itself. Note the conclusion is, "you need a detergent especially formulated for FLW to get clothes really clean." What's to say that "especially formulated" doesn't refer to a special cleaning agent to compensate for not dissolving as readily? So on and so forth - because Answer D is a SA (just one way to close the gap) rather than a NA.

    Are there other example NA questions for which the CR is a SA?
  • runiggyrunruniggyrun Alum Inactive Sage Inactive ⭐
    2481 karma
    The argument only takes you to "regular detergent doesn't dissolve in front loaders", whereas the conclusion jumps all the way to "if you want your clothes clean, you need a special detergent". D is necessary to bridge this gap (by the looks of it it's also sufficient - there's one big gap in that argument and that's about it).
    If it's not true that the detergent needs to dissolve well to get clothes clean, then you don't NEED a specially formulated detergent to get your clothes clean, because an ordinary one, even if it's sitting in clumps at the bottom of the front loader is still somehow doing its job just fine. Maybe the front loaders are super duper efficient at shaking and spinning, so they can get rid of dirt with homeopathic amounts of detergent.
  • PacificoPacifico Alum Inactive ⭐
    8021 karma
    Just remember that the correct answer to a Necessary Assumption question can be both necessary and sufficient so don't let that trip you up by discounting an answer that is both.
  • hlsat180hlsat180 Free Trial Member
    edited June 2016 362 karma
    @nye8870 @runiggyrun @Pacifico Thanks for all the responses. You made me think harder about this question because I still wasn't comfy with Answer D -- and I think I figured out why: Answer D is (correctly) a NA and not (as I originally viewed) a SA:

    Argument:
    P: O --> not DR (contrapositive: DR --> not O)
    C: RC --> not O --> S

    Answer D:
    RC-->DR

    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 & S) but only RC is linked to the premise, by Answer D (RC --> DR --> not O). The link to S (not O--> S) remains an unsupported gap.

    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." Negated, this would lead the argument to another conclusion (i.e., ordinary detergent, in lieu of special detergent, can get clothes clean in a front-loader).

    Because Answer D 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.
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