Hi AndreiGMAT,
We're asked if XY > X/Y. This is a YES/NO question. We can solve this question with a mix of TESTing VALUES and Number Properties.
1) 0 < Y < 1
Since this Fact tells us nothing about X, it's likely insufficient. We can prove this rather easily by TESTing VALUES.
IF....
X = 0
Y = 1/2
Then the answer to the question is NO.
IF....
X = -1
Y = 1/2
Then the answer to the question is YES.
Fact 1 is INSUFFICIENT
2) XY > 1
IF....
X = 2
Y = 1
Then the answer to the question is NO.
IF....
X = 1
Y = 2
Then the answer to the question is YES.
Fact 2 is INSUFFICIENT
Combined, we know...
0 < Y < 1
XY > 1
Since Y is a positive fraction, the only way for XY to be greater than 1 is if X is greater than 1. When multiplying a value greater than 1 by a positive fraction, the result is SMALLER than the initial value. When dividing a value greater than 1 by a positive fraction, the result is always BIGGER than the initial value. With these restrictions, we can deduce that XY will ALWAYS be SMALLER than X/Y, so the answer to the question is ALWAYS NO.
Combined, SUFFICIENT
Final Answer: C
GMAT assassins aren't born, they're made,
Rich