Can't type exponents, please read and note exponents:

If 5 to the 21st times 4 to the 11th = 2 times 10 to the Nth, what is N?

A) 11
B) 21
C) 22
D) 23
E) 32

Answer says B, but offers no explanation. I had guessed B (add the exponents after combining to get 20 on each side, but wrong wrong wrong!)

This is actually not as bad as it looks, here is how you approach this problem:
(let's rewrite the whole thing algebraically)

(5^21)(4^11) = (2^1)(10^n)

Now, break down the bases (ex: 4 and 10) into factors

(5^21) (2^22) = (2^1) (2^n)(5^n) -->(2^1)(2^n) =>2^1+n

This is what you end up with:

(5^21) (2^22) = (2^1+n)(5^n)

Now you just have to look at the common bases (ie. 5 and 2), equate them and then just look at the exponents alone :

5^21 = 5^n --> 21 = n

OR

2^22 = 2^n+1 --> 22=n+1 --> 21=n

beautifully done.

to the original poster:
* i'm moving this thread to the gmatprep math folder, where it belongs;
* when you post gmatprep problems, please (1) post them in the correct folder and (2) title them correctly, using the first 6-8 words of the problem statement, as stipulated in the forum rules.
thank you.