A self-service store has a single cashier to collect the purchase from customers. The purchase is made in 2 successive operations, first the cashier collects the account and then an employee packs the purchase. The duration of the operations follows an exponential distribution with means of 4 min. and 2 min respectively. Customers arrive at the checkout according to an exponential distribution with a mean of one every 12 min. a) Find the time that elapses from when a customer arrives until he is served. (10 points) b) Find how many customers are waiting to be served.