GMAT Math Practice Test #1

Answer D-

First change the base 8 and the base 4 to the same base 2.

8 = 2³ and 4 = 2²

Hence the equation can be written as

(2³)^2x-4 = (2²)^2x
2^6x-12 = 2^4x
6x-12 = 4x
2x = 12 or x =6

Key takeaway: When solving problems involving exponents, base is to be seen carefully. It is easy to simplify, as long as they have the same base. Remember following simple formulae:

a^m x aⁿ = a^m+n, a^m / aⁿ = a ^m-n , (a^m ) ⁿ = a ^{m x n} , (a^m ) ^1/n = a ^{m/ n}