LSAT Discussion Forum

Complete Question Explanation

Assumption—#%. The correct answer choice is (D)

This stimulus is difficult for many because of the numbers involved in the argument. However, as with most numbers-related arguments on the LSAT, remembering to focus on the language can help you cut through the numbers and demystify the reasoning.

The auditor is looking into truck purchases by XYZ, a construction company. Three years ago, XYZ bought 20 new trucks. Company records do not show those trucks having been sold last year. But, XYZ’s records show that it sold off all of its diesel-powered trucks last year. So, the auditor concludes, none of the 20 trucks XYZ bought three years ago were diesel-powered.

A flaw in this argument is that we have been told that the 20 trucks were purchased three years ago, and that none of those trucks were sold last year, or one year ago. But what about two years ago? In reaching the conclusion, the auditor assumes that XYZ did not sell any of the 20 new trucks at some time between their purchase three years ago and last year.

The question stem identifies this as an Assumption question. Indeed, this is a Defender style Assumption question. Our prephrase is that the correct answer choice will defend the conclusion against an attack based on the assumption that the new trucks purchased three years ago were not sold prior to last year.

Answer choice (A): The conclusion was limited to the question of whether the 20 trucks purchased three years ago were diesel-powered. There is no record of those trucks being sold last year, and it is not relevant to the conclusion whether all of the trucks that were sold last year were diesel-powered.

Answer choice (B): The purchase of used trucks is completely irrelevant to the argument. While XYZ sold off its own used trucks, nothing in the stimulus discusses the purchase of used trucks. Again, the issue addressed by the conclusion is whether the 20 trucks were diesel powered.

Answer choice (C): This answer choice is incorrect, because the question is not how many trucks company XYZ has, or how many it has purchased in the last three years. Rather, the question is whether the 20 trucks were diesel powered.

Answer choice (D): This is the correct answer choice. The reason given by the auditor for the conclusion that the trucks were not diesel powered is that they were not sold last year, when XYZ sold off all of its diesel powered trucks. However, this argument ignores the possibility that some of those trucks were sold two years ago. If that were the case, then XYZ’s sale of all its diesel trucks last year would not have included some of the 20 in question, because they were already gone.

Answer choice (E): XYZ’s continued ownership of trucks that it purchased more than three years ago is irrelevant to the auditor’s argument. The only issue is whether the trucks XYZ purchased three years ago were diesel powered.

Can you please assist me on how to correctly answer this question. I realize that this is an assumption question, and that the argument concludes that "none of the 20 trucks purchased 3 years ago were diesel powered". How is D the assumption?

Hi Smile,

The stimulus tells us that 20 trucks were purchased 3 years ago, and that there is no record of them being sold "last year", but that all the company's diesel trucks were sold last year. On this basis they conclude that none of the 20 trucks were diesel. The flaw in this argument is that the author is that there is no evidence about whether the 20 trucks were sold after their purchase, but before last year. If they were, then some of those trucks could have been diesel. The author must assume that this did not occur, in order to draw the conclusion in the stimulus.

This assumption corresponds well to answer choice D.

Hope this helps!

Beth

Hi Beth,

Thank you very much for your explanation. However, I am still not clear on how D is the assumption. if the 20 trucks were sold off after they were purchased but before last year, how is it possible that some of them could be diesel if all of the diesel trucks were sold last year? Is the stimulus assuming that the diesel trucks were sold during the period from when the 20 trucks were purchased and the last of the bunch was sold last year?

smile,

If they were diesel and sold before last year (but after purchase 3 years ago), then XYZ didn't have them last year, so when XYZ sold "all its diesel-powered trucks" last year, it didn't include what it already sold. If this were true, there would be no record of selling them last year, even though they were diesel. Because the auditor claims that none of those 20 trucks were sold last year, the auditor must be assuming that this entire scenario we're talking about isn't true - so none of the 20 was sold in the interim.

Robert Carroll

I hesitated on this question for a bit because D also assumes that out of the 20 trucks XYZ bought 3 years ago, that XYZ did not sell any non-diesel trucks in years 1-2, which is an unnecessary assumption.

I prephrased an answer that no diesel trucks were sold in years 1-2 and did a double take after not finding that answer. I might be looking at this question wrong, but maybe this question shows the LSAT makers allow some wiggle room in their necessary assumptions?

Hi, Alex,

Good analysis.

Answer choice (D) does not include any additional assumption that the argument itself does not contain. At a baseline level, the author assumes that these 20 trucks are not diesel. The author also assumes the company still owns these trucks. Both of these assumptions are implicit in the argument.

Answer choice (D) addresses the latter of the above assumptions. It leaves the former alone. In other words, answer choice (D) does not add any additional implicit assumption that the argument itself does not already contain.

The only thing we know from answer choice (D) is that XYZ did not sell those 20 trucks in years 1-2. Since they did not sell those 20 trucks in years 1-2, and since from the premises we know that they did not sell those 20 trucks last year, and since they got rid of all their diesel trucks last year, it is possible for us to reach the conclusion that these trucks were diesel powered.

Let's do the Assumption Negation test. If some of these 20 trucks were sold prior to last year, we no longer have any evidence to suggest that these trucks were not diesel-powered. In other words, with the negation of the statement in answer choice (A) we have completely vacated the evidence offered in support of the conclusion. That is why (A) is an assumption required by this argument. Without it we have nothing.

Even with the information in answer choice (D) we do not necessarily have sufficient information to prove our conclusion is valid. Maybe some of those 20 trucks got into bad wrecks and were not able to be sold, even though they were diesel-powered. Etc. You get the idea.

However, answer choice (D) does provide a baseline assumption that is required for us to have a valid argument.

Good job with your prephrase! However, test your prephrase against the Assumption Negation test. What if some diesel powered trucks were sold in the first two years? Would it still be possible that none of the 20 trucks were diesel powered? Yes, while the negation of your prephrased assumption would weaken the argument, and your prephrase would strengthen the argument, as phrased it is not necessary that the author assume that no diesel trucks were sold in the first two years; it is just necessary that the author assume that none of the 20 trucks was among any diesel truck that was sold in the first two years.

I hope this helps!

alexmcc wrote:I hesitated on this question for a bit because D also assumes that out of the 20 trucks XYZ bought 3 years ago, that XYZ did not sell any non-diesel trucks in years 1-2, which is an unnecessary assumption.

I prephrased an answer that no diesel trucks were sold in years 1-2 and did a double take after not finding that answer. I might be looking at this question wrong, but maybe this question shows the LSAT makers allow some wiggle room in their necessary assumptions?

Thanks Jonathan for the detailed response! I goofed and failed to put another "out of the 20 trucks XYZ bought 3 years ago..." clause before my prephrase, and I agree that D doesn't ensure that none of the 20 trucks purchased 3 years ago were diesel trucks. But could you talk about why it's an implicit assumption that XYZ still has to own ALL 20 trucks.

I think it would be consistent with the reasoning of the passage if XYZ sold any of the 20 trucks that were known to be non-diesel trucks in years 1-2. Answer D says we have to assume that's not the case. Like you say negating D would be "some of the 20 trucks were sold before last year", which seems fine, given all the trucks sold out of the 20 trucks were non-diesel trucks.

If D had said "None of the 20 trucks sold before last year were diesel trucks", then D would only have necessary elements.

As it is, D says "none of the 20 trucks were sold before last year", which seems to have an unnecessary element that the company didn't sell any non-diesel trucks out of the 20 trucks before last year.

So for the record I've never been this confused about diesel trucks in my life! I could very well be getting this all wrong, and you're welcome to say "Alex stop you're getting this all wrong!" Thanks again for your help so far.

No need to stop, alex, not when you're so close!

In order for the author to conclude that none of the 20 trucks were diesel trucks, he must assume that NO trucks were sold before last year. It looks to me like you might be thinking of this question as a Justify the Conclusion question, where you are looking for an answer that proves that the conclusion is true. In that case, answer D would still do the job, but it goes farther than it needs to. We could justify the conclusion just by saying that no diesel trucks from among those 20 trucks were sold in the first two years, which is where you look to be going.

But this is an Assumption question, which means that the answer needs to be something that the author must have believed in order to get to his conclusion. This author thinks his evidence is sufficient to prove that none of those 20 trucks are diesel trucks. If it turns out that even one of those trucks was sold in the prior two years, then his evidence would no longer be sufficient for his conclusion, simply because it would be POSSIBLE that is was a diesel truck! We don't have to know that it WAS a diesel truck, but only that it could have been, in order for his evidence to be insufficient. If we said "but wait a minute, dude - they sold one of those trucks two years ago!", then he would have to admit that we can no longer prove that none of the 20 trucks were diesel trucks. We would need more evidence - was the sold truck a diesel truck? - to get him back on solid ground.

Yes, it is true that the author must have assumed that if any of the 20 trucks were sold previously, they were not diesel trucks, but his argument as currently written must assume that NONE of the trucks were sold, in order to be so certain about his conclusion. Without that assumption, his argument has a hold in it that you could drive a truck through!