Hi ziyuenlau,

This question can be solved with a mix of TESTing VALUES and Number Properties.

We're asked if (M+N)^3 is an ODD number. This is a YES/NO question. Before we deal with the two Facts, here are the Number Properties to note....

IF....
(M+N) = an odd integer, then the answer to the question is YES.
(M+N) = an even integer, then the answer to the question is NO.
(M+N) = a NON-integer, then the answer to the question is NO.

1) M and N are integers

With Fact 1, there's no restriction on whether the sum of M and N is even or odd, so the answer to the question could be YES or NO.
Fact 1 is INSUFFICIENT

2) (M)(N) = 15

With this Fact, there are only two possibilities:
-M and N are both ODD (for example, 3 and 5), so the SUM would be EVEN and the answer to the question is NO.
-M and/or N are non-integers (for example 2 and 7.5), so the SUM would be a NON-INTEGER and the answer to the question is NO.

There is no way for M and N to end in a sum that is odd, so the answer to the question is ALWAYS NO.
Fact 2 is SUFFICIENT

Final Answer: B

GMAT assassins aren't born, they're made,
Rich