The manager of a market can hire either Mary or Alice. Mary, who gives service at an exponential rate of 20 customers per hour, can be hired at a rate of $3 per hour. Alice, who gives service at an exponential rate of 30 customers per hour, can be hired at a rate of $C per hour. The manager estimates that, on the average, each customer's time is worth $1 per hour and should be accounted for in the model. If customers arrive at a Poisson rate of 10 per hour, then

(a) what is the average cost per hour if Mary is hired? if Alice is hired?
(b) find C if the average cost per hour is the same for Mary and Alice.