Data Sufficiency

[GMAT math practice question]

m and n are positive integers greater than 1. Is m^n a perfect square?

1) m is an odd integer
2) n is an odd 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.

Since we have 2 variables (x and y) and 0 equations, C is most likely to be the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)
If m = 9 and n = 3, then mn = 9^3 = (3^2)^3 = 3^6 = (3^3)^2 = 27^2, which is a perfect square, and the answer is 'yes'.
If m = 3 and n = 3, then mn = 3^3 = (3)^3 = 27, which is not a perfect square, and the answer is 'no'.
Both conditions together are not sufficient, since they don't yield a unique answer.

Therefore, E is the answer.
Answer: E

In cases where 3 or more additional equations are required, such as for original conditions with "3 variables", or "4 variables and 1 equation", or "5 variables and 2 equations", conditions 1) and 2) usually supply only one additional equation. Therefore, there is an 80% chance that E is the answer, a 15% chance that C is the answer, and a 5% chance that the answer is A, B or D. Since E (i.e. conditions 1) & 2) are NOT sufficient, when taken together) is most likely to be the answer, it is generally most efficient to begin by checking the sufficiency of conditions 1) and 2), when taken together. Obviously, there may be occasions on which the answer is A, B, C or D.