Quant, Data Sufficiency questions

Question

Hi, I am new to GMAT. I have a question regarding Data Sufficiency questions as part of quantitative section. Given that the goal to these kinds of questions is to find YES or NO answers to questions, should any of the statements (together or alone) that prove the answer to the question is NO with certainty, would be the correct answer?

For example, for the questions:

Is X< 4?

1) Statement 1: X2 <36   
2) Statement 2: X - 2 =8

Based on statement 1: X can be any number less than 6 which may possible include numbers below 4, but X can also be 5 which is greater than 4. Accordingly, this statement does not offer enough evidence to suggest that X<4 in all cases.

Based on statement 2: X = 10 which is bigger that 4, as such it provides answer NO to the question with full certainty.

Does this mean that statement 2 alone is the correct answer?

logro · Answer

Hey Tfgal,

You are right on the approach front

Statement 1: Implies that -6<x<6 (Don't forget this when you are dealing with the squares)
So we have 4 and 5 as possible values of x which would render the question stem invalid

Statement 2: X-2=8 gives us X=10 which is always greater than 4. Hence Sufficient

For DS questions you should always look for the question stem to be true under every circumstance

Tfgal · Answer

logro

Thanks logro...

Yes I just wanted to confirm that if a statement proves the question stem wrong in all conditions or in the other words provides NO as an answer to it in all circumstances, would that be the correct answer.

Although I conveyed my point but it seems that I made very silly mistakes in the example I made up! I think I am too tired and need to go to bed ASAP!

Thanks a lot for your help

AndrewN · Answer

Hello,  Tfgal . Just remember that when it comes to DS questions, your goal is to ascertain whether you could answer the question  with certainty . It might seem counterintuitive, but a definitive NO to the question does indeed satisfy the  sufficient  part of a data  sufficiency  question. You are not alone in feeling somewhat confused about this notion. I have worked with plenty of students who, upon going back over their mistakes, have discovered that their brain automatically turned a NO into a NOT SUFFICIENT. Practice helps counter this misleading intuition. For each statement, your goal should be to determine whether the answer is yes or no  on its own . (Some students even find it useful to write Y/N for  each  statement at first.) If they both work, of course, then you have to abandon that statement and move on to the other.

Another thing to keep in mind with DS questions is that the two statements always work in tandem to provide consistent information, whether one statement or the other actually proves sufficient. That is, you will never find an official DS question in which statements (1) and (2) provide conflicting information. Knowing this can help you reassess whether, in certain cases, you may have confused one NO with another.

Best of luck with your studies.

- Andrew

Tfgal · Answer

Dear Andrew,
Thanks for your awesome note. Your explanation is really helpful and I highly appreciate your time and attention.
???

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Quant, Data Sufficiency questions

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