Hi littlegirl,
Since the prompt restricts X and Y to POSITIVE numbers (NOT necessarily positive INTEGERS though), we're going to be dealing with Number Properties that interact with Exponents. You'll likely want to TEST VALUES and keep an open mind about the possible positive values that you can use...
We're asked if X^4 < Y^3. This is a YES/NO question.
Fact 1: X^5 < Y^3
IF....
X = 1
Y = 2
1 < 8
1^4 is < 2^3 and the answer to the question is YES.
IF....
X = 1/2
Y = 1/3
1/32 < 1/27
(1/2)^4 is NOT < (1/3)^3 and the answer to the questions is NO
Fact 1 is INSUFFICIENT
Fact 2: X^4 < Y^5
IF....
X = 1
Y = 2
1 < 32
1^4 is < 2^3 and the answer to the question is YES.
IF....
X = 3
Y = 4
81 < 1024
3^4 is NOT < 4^3 and the answer to the question is NO.
Fact 2 is INSUFFICIENT
Combined, we know..
X^5 < Y^3
X^4 < Y^5
IF....
X = 1
Y = 2
1 < 32
1^4 is < 2^3 and the answer to the question is YES.
To get a NO answer, we would have to think about using fractions (as we did in Fact 1). However, fractional values are not possible because the two given Facts, when combined, won't support a set of fractional answers.
To get a NO answer, we would need X^4 to be >= Y^3.
Using fractions, this would require that X >= Y.
As we raise X and Y to increasing 'powers', their respective values would DECREASE (since multiplying a fraction by another fraction makes the product SMALLER).
The two Facts give us one restriction is which X has the higher exponent and another restriction in which the Y has the higher exponent. There's no way for both of these restrictions to exist AND for X^4 to be > Y^3.
This is a very long way of saying that, when combined, the answer to the question is ALWAYS YES.
Combined, SUFFICIENT
Final Answer:
GMAT assassins aren't born, they're made,
Rich