About 4% of the citizens of the United States have type AB blood. If an agency needed type AB blood and donors came in at random, find the average number of donors that would be needed to get a person with type AB blood.

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