Logical reasoning PrepTest 130 · Section 3 · Question 5

Incorrect. If anything, this suggests that the cheaper stuff works just as well, as the experts use it for their own needs, supporting the view that it's the better buy.

Incorrect. This answer is tempting because it seems to suggest alternative purposes for the motor oil. However, it doesn't necessarily do that—these other tests could serve as a proxy for engine wear. For example, if engine wear is related to gas mileage, then testing for that measurement could still speak to engine wear. As such, this answer doesn't bring up alternative purposes and thus doesn't weaken the argument.

Incorrect. This answer would apply to all motor oils, so this answer doesn't call into question the value of the cheaper oil.

Incorrect. First, this answer is about "some" cars, which is unlikely to have much of an impact on an argument that speaks generally. Second, there's no indication of the cost of this certain brand of oil.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
Motor oil slows engine wear
Study - Various brands of motor oil slowed engine wear equally well
Conclusion - Cheaper motor oil brands are a better buy

Answer Anticipation:
The conclusion here draws a comparison—cheaper motor oil brands are a "better" buy. Why does it believe that? Because they're cheaper and work "equally well" at slowing down the wear on the engine. If something works just as well but is cheaper, then it would, in fact, be a better buy.

So to weaken this argument, we would need to show that the cheap stuff doesn't work just as well at that lower cost. Since the cost isn't up for debate, it must be the effectiveness of this motor oil.

The study looked at taxis—which would be driven more frequently and thus highlight any shortcomings in a product meant to slow engine wear—and saw the various oils working to slow down that wear just as well as each other, so the effectiveness at slowing down engine wear isn't in question. And while the stimulus notes that motor oil serves to slow down this wear, it doesn't state that this is the only reason that people use motor oil. If there are other reasons, and the more expensive stuff is better for those reasons, then the cheap stuff may not be the better buy.

Answer Explanation:
This answer brings up that there are other things motor oil does to keep an engine running than just slow down wear. If that's the case, then the ability of the cheap oils to slow down that wear as well as the expensive stuff doesn't necessarily mean that it's a better deal, as the more expensive stuff might serve these other purposes better.

Key Takeaway:
When an argument draws a comparison in the conclusion, make sure that it has established the criteria by which that determination can be made. Here, the better value is definitely related to cost, but it must also be related to effectiveness. However, the stimulus never established that effectiveness was solely related to the ability to slow down engine wear, thus failing to establish the appropriate criteria to determine value.