d$.)$ Last National is considering adding tellers at the current drive$-$in facility. It has decided on $$5$ per hour as the imputed cost of customer waiting time in the system. The hourly cost of a teller is $$10.$ The average arrival rate of customers has reached $30$ per hour. On the basis of the total hourly cost of tellers and customer waiting, how many tellers do you recommend? Assume that with use of pneumatic tubes, tellers can serve customers as though there were a single queue.