Data Sufficiency

[GMAT math practice question]

Is pqr is a multiple of 5?

1) p, q and r are consecutive odd integers.
2) p, q and r are prime numbers.

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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 3 variables (p, q and r) and 0 equations, E 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):
3, 5 and 7 is the unique triplet of three consecutive odd integers which are prime numbers, and 3*5*7 is a multiple of 5.
Thus, both conditions together are sufficient.

Since this question is an integer question (one of the key question areas), CMT (Common Mistake Type) 4(A) of the VA (Variable Approach) method tells us that we should also check answers A and B.

Condition 1)
If p = 3, q = 5 and r = 7, then pqr = 105 is a multiple of 5, and the answer is 'yes'.
If p = 7, q = 9 and r = 11, then pqr = 693 is not a multiple of 5, and the answer is 'no'.
Condition 1) is not sufficient since it doesn't yield a unique answer.

Condition 2)
If p = 3, q = 5 and r = 7, then pqr = 105 is a multiple of 5, and the answer is 'yes'.
If p = 3, q = 7 and r = 11, then pqr = 231 is not a multiple of 5, and the answer is 'no'.
Condition 2) is not sufficient since it doesn't yield a unique solution.

Therefore, C is the answer.
Answer: C

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.