Hi crackgmac,
I would actually go with plugging in numbers here.
But the question is asking for something that MUST be divisible by 3. If we plug in numbers into an answer choice and it works, we have only proven that the expression in the answer choice COULD be divisible by 3 (not that it MUST be).
But if we plug in numbers and it doesn't work, we have proven that it does NOT HAVE TO BE divisible by 3, and so can eliminate.
In other words, here, you can plug in numbers to eliminate choices (you can prove that they could be false--that they don't HAVE to be divisble by 3). But if you plug in a number and it works, you have only proven that it could be true--that it could divide 3, not that it must.
So you simply plug in numbers until you are able to elminate all the answer choices. Again, here, you can't plug in numbers to select an answer choice (if it works, you keep it in the running, and if it doesn't you eliminate).
Remember that the best approach on the GMAT is not necessariy the same as some "correct" approach you would have to us in a show-your-work exam. The testmaker wants to reward testtakers who use alternative and more efficient reasoning techniques.
duong--b/c this is a released GMAT question, it cannot be "wrong".
If the question is a released GMAT question or from a major test prep company, you should always assume the question is "right".
Kaplan Teacher in Toronto
