QUESTION TEXT: Auditor: XYZ, a construction company, purchased 20…
QUESTION TYPE: Necessary Assumption
CONCLUSION: None of the trucks purchased 3 years ago were diesel powered.
REASONING: The company purchased 20 trucks 3 years ago. They sold none of those trucks this year, but they did sell all of their diesel trucks this year.
ANALYSIS: I found this question very difficult, and had to read it three times then come back to it. If the company sold all its diesel trucks last year, how could it not have sold any of the 20 trucks 3-year old if they were diesel powered?
It turns out I made a simple oversight. What if the company sold some of the 20 trucks last year, or two years ago?
I’ll give a numerical example. Let’s say the company owned five diesel trucks of type X. Three years ago, they bought four diesel trucks of type D. They also own two regular trucks, type T.
Two years ago, they sell all D trucks. And last year year they sell all their remaining diesel trucks, the X’s. Here’s the stock of trucks at the end of each year:
Four years ago: XXXXX TT
Three years ago: XXXXX DDD TT
Two years ago: XXXXX DDD TT
Last year: XXXXX TT
This year: TT
So in this example, the company did buy diesel trucks three years ago (DDD). It didn’t sell them last year, but only because it already sold them two years ago.
___________
- Not required.
Negation: One truck sold last year was gasoline powered, the rest were diesel. - Who cares what used trucks the company bought? The stimulus is about the 20 new trucks.
Negation: The company also bought a used truck 3 years ago. - It doesn’t matter whether the company bought other trucks. We only care about the 20 trucks bought three years ago.
Negation: Two years ago, the company bought a new gasoline powered truck. - CORRECT. If some trucks were sold two years ago, then they could have been diesel trucks even though they weren’t sold last year.
Negation: The company sold some of the 20 trucks earlier than last year. - We don’t care about trucks purchased more than 3 years ago. The question is about trucks purchased exactly 3 years ago.
Negation: The company still has a truck it bought 18 years ago. Wowza!
Recap: The question begins with “Auditor: XYZ, a construction company, purchased 20”. It is a Necessary Assumption question. To practice more Necessary Assumption questions, have a look at the LSAT Questions by Type page.
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Price says
For answer choice D., couldn’t the company have sold some of its non diesel trucks before last year? I can see how it would be required that none of the 20 trucks (that were diesel) were sold before last year.
FounderGraeme Blake says
Suppose you had 100 diesel trucks three years ago, and bought 20 more. Then two years ago, you sold the 20 new ones. Then last year you sold the 100 original diesel trucks. So this year you have no trucks on hand.
The argument is premised on the company **still having** the trucks. The idea is that any truck which got through the diesel sale last year is a non-diesel truck. This is a good argument, but only if those same trucks weren’t previously sold – the company needs to still have them.
Linds says
Hi, I am a bit confused on why Grame is showing the negation of the answer choices. I see this on a lot of his explanations. Is this a strategy I should be using and, if so, why? How does this help me answer the question?
FounderGraeme Blake says
It’s for necessary assumption questions. On NA questions, if you negate the right answer the argument falls apart. Whereas negations of the wrong answers don’t wreck the argument. You can use the ones I wrote to practice if you’ve negated correctly.
This article has more info! https://lsathacks.com/lsat-negations/
Tom says
Tough question that focuses not on logic, but on an easily overlooked detail. I also got this wrong on first blush.
I think one good take-away is don’t jump to the answer choices straight away or you’ll get twisted and turned around. Find the necessary assumption yourself. Rephrase and understand it before you touch the choices.
Doing the negation test for every answer and mulling over how it impacts the conclusion should be a last resort, since it takes a long time and brain power.
TutorLucas (LSAT Hacks) says
I agree that it’s a good idea to go into the answer choices with a strong sense of the conclusion, premises, reasoning, and gap between the conclusion and premises. The only thing I’d add is that it’s important to not get too caught up in the necessary assumption(s) you’ve pre-phrased. Sometimes students eliminate answer choices on the basis of them not matching their pre-phrase, and end up missing the correct answer because the necessary assumption that LSAC chose to focus on was quite subtle. So, I’d just suggest prioritizing a strong sense of the reasoning of the argument over coming up with a necessary assumption pre-phrase. If you do have one, that’s great, but try to remain open to the possibility that it doesn’t match the answer choices, and don’t be too hasty to eliminate answer choices if they don’t match.
The negation test can be a very good second check of whether you’ve found the correct answer, but yes, it is better to try to find the necessary assumption without it first, and then apply the test.