If x and y are integers and xy does not equal 0, is xy < 0?

(1) y = x^4 – x^3

(2) x is to the right of y on the number line

My Answer was E . Below is my reasoning.

1) X could be +ve or -ve & Y is positive. So product is either +ve or -ve . Not sufficinet
2) X > Y , so X & Y could be +ve, -ve / +ve +ve / -ve -ve, so product could be +ve or -ve. Not sufficient.
3) Taking both together,
X>Y, Y = X^4-X^3. This condition is never possible, because, for any value of X, Y is always > X.
X =2, Y = 8, X=-2 Y =24 ... etc ..
Not sufficient.

But, MGMAT answer is C, and the reasoning is

(1) AND (2) SUFFICIENT: Because statement (1) tells us that y must be positive, and statement (2)
tells us that x is greater that y, we know that x also is positive. Therefore we know that xy is not less than 0.

In GMAT data sufficiency, when evaluating choice C, should both conditions be evaluated to verify if they
can exist and together or just get the answer without any evaluation?

You are the second person in about a week to question this one. Thanks for the attention to detail. We will definitely fix this one.

You may be interested in the related thread:
http://www.manhattangmat.com/forums/pos ... html#19479

_________________
Emily Sledge
Instructor
ManhattanGMAT