manhattan gmat exponents

[resilient]

If 4^4x=1600, what is the value of (4^x-1)^2?

qa is 5/4

[simplyjat]

Are you sure you got the question right...
1. 4^(4x) can never be 1600....
2. (4^x-1)^2 can never yield a fraction

[opengmat]

If we look at the problem asked to solve:

(4^x-1)^2 = 4^(2x - 2) = 4^2x / 16 ....... (a) simplifying

Now looking at the inputs:

4^4x = 1600
if we sq both sides, we get

=> 4^2x = 40 ........(b)

=> So subs. in (a) from (b), 4^2x/16 = 40/16 = 5/2 [check your answer, 5/4 is correct ?]