1. ## probability problem

n balls are randomly distributed among three boxes. Find the probability of
(a) exactly two empty boxes;
(b) exactly one empty box
(c) no empty boxes.

so the answers are (a) 1/3^(n-1)
(b) [(2^n)-2]/3^(n-1)
(c) [3^(n-1)-2^n+1]/3^(n-1)

I don't understand why the denominator is 3^(n-1) and why the numerator for (b) is 2^n-2?