2015 Official Guide Data Sufficiency #172

[mattCFA2]

Is n an integer?
(1) n^2 is an integer

I said sufficient, because I thought of possibilities such as 1, 2, and 1/2 wouldn't work.
For statement 1, how on earth would anyone be expected to consider radical 2? It just seems that that would be so unrealistic to consider in the heat of the battle.
Are there any tips or rules I should be considering when I evaluate questions like this?
Any expert advice is much appreciated!

Matt

[[email protected]]

Hi mattCFA2,

DS questions test a variety of skills (not just "math" skills), including thoroughness, accuracy, attention to detail, your ability to prove that something consistent is happening, etc. and certain DS questions aren't really about doing math at all.

Here, we're asked if N is an integer. That is a very specific YES/NO question.
For example, If N = -2, -1, 0, 1, 2, 3, 4, 5, etc., then the answer to the question is YES.
For example, if N = -4.2, -1/2, +2/3, 117.3, then the answer to the question is NO.

Fact 1: N^2 is an INTEGER.

So, N^2 is a WHOLE NUMBER...

N^2 COULD = 0, 1, 2, 3, 4, 5, 6, etc.

If N^2 = 1, then N = 1 and the answer to the question is YES
If N^2 = 2, then N = (root2) and the answer to the question is NO.

I didn't have to go two far to find those results - I just TESTed the first two positive integers. Since the results are inconsistent (notice how I barely had to do any math), then Fact 1 is INSUFFICIENT.

From a tactical standpoint, the best general advice that I can offer is to be a little cynical. Finding the "first TEST" won't be hard, but think about what else is possible (and sometimes you might have to "play around" a bit to find that alternative result).

GMAT assassins aren't born, they're made,
Rich

[Brent@GMATPrepNow]

Is n an integer?
(1) n^2 is an integer

I said sufficient, because I thought of possibilities such as 1, 2, and 1/2 wouldn't work.
For statement 1, how on earth would anyone be expected to consider radical 2? It just seems that that would be so unrealistic to consider in the heat of the battle.
Are there any tips or rules I should be considering when I evaluate questions like this?
Any expert advice is much appreciated!

Matt

This is a common trap on the GMAT.
Unless told otherwise, you need to consider integer values, negative/positive values, rational (fractional) values and even irrational values (like √2)

The mistake you made is covered in our free videos titled "Avoiding Common Mistakes - Part I" and "Avoiding Common Mistakes - Part II"

You can find them here: https://www.gmatprepnow.com/module/gmat-data-sufficiency

Cheers,
Brent