An 8-sided die is rolled. The sides are numbered 1 through 8. Find the average number of rolls it takes to get two 5s.

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