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# Isn't this a bad DS question?

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Intern
Joined: 06 Feb 2019
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25 Jul 2019, 04:11
Find the percentage change in the volume of cylinder
1, diameter (d) is increased by 20%,
2. height is increased by 21%

I read that DS statements will lead to the same answer separately, this question is confusing because we don't know if each implies the unstated variable (height and radius respectively) are changed or not.
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26 Jul 2019, 07:15
unebaguette wrote:
Find the percentage change in the volume of cylinder
1, diameter (d) is increased by 20%,
2. height is increased by 21%

I read that DS statements will lead to the same answer separately, this question is confusing because we don't know if each implies the unstated variable (height and radius respectively) are changed or not.

Nope, this problem is fine. If you don't know whether the unstated variable is changed, that just means the statement is insufficient. In this case, (1) and (2) are each insufficient alone (since neither one tells you whether the other variable was changed) but the statements are sufficient when put together. The answer would be C.

What you're thinking of, is a rule of thumb that says the statements will never directly contradict each other. That is, it'll never be impossible for statements 1 and 2 to both be true at the same time. For example, this is a bad DS problem:

What is the value of x?

(1) x is a multiple of 4.
(2) x is prime.

x can't be both a multiple of 4 and prime, so this is a problem you would never see on the GMAT.
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26 Jul 2019, 08:22
ccooley wrote:
unebaguette wrote:
Find the percentage change in the volume of cylinder
1, diameter (d) is increased by 20%,
2. height is increased by 21%

I read that DS statements will lead to the same answer separately, this question is confusing because we don't know if each implies the unstated variable (height and radius respectively) are changed or not.

Nope, this problem is fine. If you don't know whether the unstated variable is changed, that just means the statement is insufficient. In this case, (1) and (2) are each insufficient alone (since neither one tells you whether the other variable was changed) but the statements are sufficient when put together. The answer would be C.

What you're thinking of, is a rule of thumb that says the statements will never directly contradict each other. That is, it'll never be impossible for statements 1 and 2 to both be true at the same time. For example, this is a bad DS problem:

What is the value of x?

(1) x is a multiple of 4.
(2) x is prime.

x can't be both a multiple of 4 and prime, so this is a problem you would never see on the GMAT.

See that's what I thought, that it was C, but the answer is D, that each answer is sufficient.

Your certainty this is a fine question but getting the "wrong" answer as well in addition to the fact that "on the GMAT, two data sufficiency statements always provide TRUE information and these statements never contradict each other" leads me to conclude this is not a good question.
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26 Jul 2019, 08:29
ccooley wrote:
unebaguette wrote:
Find the percentage change in the volume of cylinder
1, diameter (d) is increased by 20%,
2. height is increased by 21%

I read that DS statements will lead to the same answer separately, this question is confusing because we don't know if each implies the unstated variable (height and radius respectively) are changed or not.

Nope, this problem is fine. If you don't know whether the unstated variable is changed, that just means the statement is insufficient. In this case, (1) and (2) are each insufficient alone (since neither one tells you whether the other variable was changed) but the statements are sufficient when put together. The answer would be C.

What you're thinking of, is a rule of thumb that says the statements will never directly contradict each other. That is, it'll never be impossible for statements 1 and 2 to both be true at the same time. For example, this is a bad DS problem:

What is the value of x?

(1) x is a multiple of 4.
(2) x is prime.

x can't be both a multiple of 4 and prime, so this is a problem you would never see on the GMAT.

I included a screenshot and the source: https://www.test-guide.com/gmat/free-gm ... est-3.html

I know you're an "expert" but unfortunately, i don't trust your judgment anymore after your insisting that this is a good question but also getting the "wrong" answer (I agree with you that it should be C)

This is a bad question, and given their explanation, and also yours, the official gmat should not have questions this confusing or open to misinterpretation.
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26 Jul 2019, 09:07
It is possible for a question to be fine but for the solution to be bad. That solution is absolutely terrible - it's wrong about the math (if the diameter increases by 20%, the radius also increases by 20%, and not by 10% as that solution incorrectly claims), and it gets the answer to the question wrong too, at least as I'd interpret the problem (I'd interpret it the same way ccooley does). But the wording of the problem itself is so imprecise, there's no way to guess what it's even asking. What's happening to this cylinder? Is the height changing? Is the radius changing? The question doesn't tell us anything about the situation we're meant to answer a question about.

So yes, the question and solution are both bad, and I would strongly encourage you to find better study materials.
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27 Jul 2019, 12:28
I think you're both being a bit unclear. I think the question could be clearer as well if it said ONLY the diameter is changed and ONLY the height is changed. This is what leaves is up for interpretation, making the question unclear.
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Updated on: 13 Aug 2019, 05:40
You are correct that DS statements must be consistent.

The question did not specify what happened to height in (1) and diameter/radius in (2). You may be thinking that the 20% and 21% changes are inconsistent, but each happens to different variables. So there is no contradiction.

I’m the combined 1 and 2,
V= pi r^2 h
Substitute the new r and h and divide by the original volume to get the new percent change.

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Originally posted by TestPrepUnlimited on 10 Aug 2019, 06:29.
Last edited by TestPrepUnlimited on 13 Aug 2019, 05:40, edited 3 times in total.
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10 Aug 2019, 07:02
TestPrepUnlimited wrote:
It is a bad question in the sense that is it inconsistent. Diameter increase by 20% -> radius increase by 20% and height increase by 21% are different operations. We need to change the 20% to 10% or 21% to 44% for this to make sense. Wording wise we can assume nothing else changed except for the stated ones.

No, this is not correct. There is nothing inconsistent about the statements in this question, as long as you assume both the radius and height can change, which is the only logical assumption one could make in a question like this (since it's the only way for the statements to be consistent at all). You seem to be suggesting that the question would have consistent statements if it were changed to this:

By what percent did the volume of a cylinder change?
1. The radius increased by 10% (and the height stayed the same)
2. The height increased by 21% (and the radius stayed the same)

But those statements are not consistent. Those statements give the same answer to the original question, but that's not what "consistent" means. Consistent means "can both be true", and it cannot simultaneously be true that the height stays the same, as Statement 1 tells us, and that the height increases by 21%, as Statement 2 tells us. This question would make no sense as a GMAT DS question.
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13 Aug 2019, 05:34
You’re correct Ian. My assistant was too hurried in their explanation.

Posted from my mobile device
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14 Aug 2019, 02:46
I think it good for the brain
Re: Isn't this a bad DS question?   [#permalink] 14 Aug 2019, 02:46
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