Isn't this a bad DS question?

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.

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.

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.

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.

The answer is C

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.

You’re correct Ian. My assistant was too hurried in their explanation.

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