Cars arrive at a toll plaza at the rate of 60 cars per hour. There are four booths, each capable of servicing 30 car per hour. Assume a Poisson distribution for arrivals and an exponential distribution for service time. If it costs $17.20 per hour to operate a toll booth and the waiting cost per car is $0.25 per minute spent at the plaza, how many booths should be operated to minimize total cost?