an urn contains K balls, of which B are black and K-B are white. fred drasw a ball at random and replaces it, N times. What is the expection of the number of times a black ball (denote as n_b) is drawn?

the answe is expection = N (B/K) but i cant get it.

what i did was expection= sum of all possible n_b X probability distribution of n_b

which in this case is a binomial distribution.