Of the volunteers donating blood in a clinic, 80% have the Rhesus (Rh) factor present in their blood.
a If five volunteers are randomly selected, what is the probability that at least one does not have the Rh factor?
b If five volunteers are randomly selected, what is the probability that at most four have the Rh factor?
c What is the smallest number of volunteers who must be selected if we want to be at least 90% certain that we obtain at least five donors with the Rh factor?