Max@Math Revolution wrote:[GMAT math practice question]

Is the positive integer x divisible by 60?

1) x is divisible by 6
2) x is divisible by 10

Statements combined:
Case 1: x=30, with the result that x is divisible by both 6 and 10
Here, x is NOT divisible by 60, so the answer to the question stem is NO, ..
Case 2: x=60, with the result that x is divisible by both 6 and 10
Here, x IS divisible by 60, so the answer to the question stem is YES.
Since the answer is NO in Case 1 but YES in Case 2, the two statements combined are INSUFFICIENT.

The correct answer is C.

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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.

The first step of the VA (Variable Approach) method is to modify the original condition and the question, and then recheck the question.
The question asks if x = 60t for some integer t.
x = 60t
⇔ x = 2^2*3*5*t


Conditions 1) (x = 2*3m for some integer m) and 2) (x = 2*5n for some integer n) do not tell us whether 2^2 is a factor of x. So, they are not sufficient, when taken together. For example, if
x = 60, then the answer is 'yes', but if
x = 30, then the answer is 'no'.

Both conditions together are not sufficient, as the question does not have a unique answer.

Therefore, E is the answer.

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.

Answer: E