Data Sufficiency

[Math Revolution GMAT math practice question]

Is n an integer?

1) 3n is an integer
2) 4n is an integer

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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.

Condition 1)
If n = 1, the answer is 'yes'.
If n = 1/3, the answer is 'no'.
Since we don't have a unique solution, condition 1) is not sufficient.

Condition 2)
If n = 1, the answer is 'yes'.
If n = 1/4, the answer is 'no'.
Since we don't have a unique solution, condition 2) is not sufficient.

Conditions 1) & 2)
Since n = 4n - 3n is a difference of two integers, n is also an integer.
Conditions 1) and 2) are sufficient, when considered together.

Therefore, C is the answer.

Answer: C

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.