Diphtheria During the past 34 years, there were 56 cases of diphtheria in the United States.

a. Find the mean number of cases of diphtheria per year. Express the result with five decimal places.

b. Find the probability of no cases of diphtheria in a year.

c. Find the probability that the number of diphtheria cases in a year is 5 or fewer. If a year has more than 5 cases of diphtheria, is that a significantly high number?