Cards are drawn at random from a deck and replaced after each draw. Find the average number of cards that would be drawn to get 4 face cards.

Another type of problem that can be solved uses what is called the negative binomial distribution, which is a generalization of the binomial distribution. In this case, it tells the average number of trials needed to get k successes of a binomial experiment. The formula is
μ = p/k
where k = the number of successes
p = the probability of a success