there are 2 kind of questions in data sufficiency:

- the one that ask to find the result/number of a problem
- the one where you have to answer YES or NO

the way i approach it is that if it is a YES or NO one i write Y N at the top of my notebook. it's very helpful.

once u ve actually determined this then the DS questions are about "do you have sufficient data to: find the result/answer YES or NO" ?

in the first case if you have sufficient data to find the number, then is ok

in the second case you need sufficient data either for answering Y or for answering NO. but once you have sufficient data, no matter if for saying Y or N, then is ok.

both in the first and in the second case, it is NOT ok when you DONT have sufficient data to give an answer


getting back to the way you posed the problem, your "understanding" is right



i'm sure you can find much better of answers than mine in this forum and googling GMAT DATA SUFFICIENCY

Thanks for the reply, Ian. I read that exactly from the GMAT Official Review as the Official is the only practice test that I try so far, and it pissed me off. It's sufficient enough for an answer and it's a NO, but the answer says not sufficient because the answer is wrong. I don't remember the question number, but it has brought to my attention.

@Ian

There is, for example, only one such question in the Official Guide.

Can you pls tell which question you are talking about.

Thanks
Amit