A firm has monitored the duration of long-distance telephone calls placed by its employees to help it decide which long-distance package to purchase. The duration of calls was found to be exponentially distribution, with a mean call length of 5 minutes.
a) What is the probability a call lasts more than 2 minutes?
b) What is the probability a call lasts between 5 and 8 minutes?
c) If a call has lasted 3 minutes, what is the probability it lasts an additional 4 minutes?