A travel agency receives an average of 150 calls per hour (time between calls are exponentially distributed). It takes an operator an average of 5 minutes to handle a call (exponentially distributed). If a caller gets a busy signal, the travel agency assumes that he or she will call a competitor, and the travel agency will lose an average of $50 in profit. The cost of keeping a phone line open is $12 per hour. How many operators should the travel agency have on duty