Calls arrive at a rate of $150$ per hour to the $800$ number for the Land$$s Beginning mail$-$order catalog company. The company currently employs $20$ operators who are paid $$10$ per hour in wages and benefits and can each handle an average of $6$ calls per hour. Assume that interarrival times and service times follow the exponential distribution. A maximum of $20$ calls can be placed on hold when all the operators are busy. The company estimates it costs $$25$ in lost sales whenever a customer calls and receives a busy signal.