Data Sufficiency

[GMAT math practice question]

Is n an even number?

1) n(n+1) is an even number
2) n(n+2) is an even number

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Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

Since we have 1 variable (n) and 0 equations, D is most likely to be the answer. So, we should consider each of the conditions on their own first.

Condition 1):
All integers satisfy condition 1) since n(n+1) is the product of two consecutive integers:
If n = 1, n(n+1) = 1*2 = 2 is an even number, but n is odd, so the answer is 'no'.
If n = 2, n(n+2) = 2*3 = 6 is an even number and n is even, so the answer is 'yes'.
Since we do not have a unique answer, condition 1) is not sufficient.

Condition 2)
The product of two integers can only be even if at least one of them is even. Since n and n+2 must both be even, or both be odd, n must be even.
Thus, condition 2) is sufficient.

Therefore, B is the answer.

Answer: B

If the original condition includes "1 variable", or "2 variables and 1 equation", or "3 variables and 2 equations" etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A,B,C or E.