A repairman is to be hired by a company to repair machines that break down following a Poisson process, with an average rate of four per hour. The cost of non-productive machine time is Rs 90 per hour. The company has the option of choosing either a fast or a slow repairman. The fast repairman charges Rs 70 per hour and will repair machines at an average rate of 7 per hour, while the slow repairman charges Rs 50 per hour and will repair machines at an average rate of 6 per hour. Which repairman should be hired?