Suppose that the length of long distance phone calls, measured in minutes, is known to have an exponential distribution with the average length of a call equal to eight minutes.

a. Define the random variable. X = ________________.

b. Is X continuous or discrete?

c. μ = ________

d. σ = ________

e. Draw a graph of the probability distribution. Label the axes.

f. Find the probability that a phone call lasts less than nine minutes.

g. Find the probability that a phone call lasts more than nine minutes.

h. Find the probability that a phone call lasts between seven and nine minutes.

i. If 25 phone calls are made one after another, on average, what would you expect the total to be? Why?