GMATPrep - value of n

[California4jx]

Its seems very simple but I couldnt get to the right answer - pls share solution

OA: B

[deepamohn]

5^21 * 4^11= 2 * 10^n

5^21 * 2^22 = 2* 2^n * 5^n
5^21 * 2^22 = 2^n+1 * 5^n

n+1=22
n = 21