Data Sufficiency

[Math Revolution GMAT math practice question]

Is n an odd number?

1) n is the sum of 2 prime numbers
2) n is a multiple of 11

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

Is n an odd number?
1) n is the sum of 2 prime numbers
2) n is a multiple of 11

Is n odd ?

(1+2) Insufficient:

> Take n = 22 = 3+19 to answer in the negative
> Take n = 33 = 2+31 to answer in the positive

The above follows the notations and rationale taught in the GMATH method.

Regards,
fskilnik.

=>
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)
If n = 2 + 3, then n = 5 is odd and the answer is 'yes'.
If n = 3 + 5, then n = 8 is even and the answer is 'no'.
Since we don't have a unique solution, condition 1) is not sufficient.

Condition 2)
If n = 11, then n is odd and the answer is 'yes'.
If n = 22, then n is even and the answer is 'no'.
Since we don't have a unique solution, condition 2) is not sufficient.

Conditions 1) & 2)
If n = 2 + 31, then n = 33 is odd and the answer is 'yes'.
If n = 13 + 31, then n = 44 is even and the answer is 'no'
Since we don't have a unique solution, both conditions 1) and 2) are not sufficient, when taken together.

Therefore, E is the answer.

Answer: E
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.