Data Sufficiency

alex.gellatly wrote:What is the value of [x-y]^4?

1. The product of x and y is 7
2. x and y are integers

Thanks

(1) xy = 7
If x = 1, y = 7, then (x - y)^4 = (1 - 7)^4 = (-6)^4
If x = 1/7, y = 7, then (x - y)^4 = (1/7 - 7)^4 = fraction
No definite answer; NOT sufficient.

(2) x and y are integers.
There can be infinite values of (x - y)^4; clearly NOT sufficient.

Combining (1) and (2), we know that xy = 7, such that x and y are integers.
Possible combinations are: (x = 1, y = 7), (x = 7, y = 1), (x = -1, y = -7), (x = -7, y = -1)
All these combinations give the same value for (x - y)^4 as (6)^4 or (-6)^4, which gives the same value; SUFFICIENT.

The correct answer is C.