5. The exponential distribution is frequently applied to the waiting times between successes in a Poisson process. If the number of calls received per hour by a telephone answering service is a Poisson random variable with parameter λ = 6, we know that the time, in hours, between successive calls has an exponential distribution with parameter β = 1/6. (a) Write down the density function for the needed exponential distribution. (b) What is the probability of waiting more than 15 minutes between any two successive calls?