exponential distribution

On Saturday morning customers arrive at the Armstrong Nursery at the average rate of 25 per hour. There are 8 employees who assist customers in selecting their plants. Each employee spends an average of 15 minutes with a customer. Following plant selection, customers then go to pay for their merchandise. There are two cashiers, and each can check out an average of 2 0 customers an hour. Once the merchandise has been paid for, customers either carry out their purchase or it is delivered to their car. Twenty percent of customers carry out their purchase. There are 3 workers who do the delivery, and each delivery requires an average of 5 minutes. Determine the average time a customer spends at Armstrong Nursery on a Saturday morning if the customer has the purchase delivered to his car. Assume that customers arrive according to a Poisson distribution and that the service times follow an exponential distribution.