A new full-service, attendant-staffed fuel and car service station on Interstate 95 will service only northbound cars. The manager estimates that customers will arrive every 4 minutes and will require 6 minutes to be served at the pump. How many pumps should be installed if the manager desires a utilization factor of .75? What percentage of time will the pumps be idle? Assume that the coefficients of variation of both the inter-arrival and the service times are equal to 1. Please show formula's with explanations.