Presently, one operator is tending five identical machines. Each machine is used to produce similar products. With five machines it has been observed that during a cycle a total of 20 machine-minutes are consumed by machines waiting for service to begin. In the past, it was observed that the operator was idle for 10 minutes each cycle when assigned to tend three machines. Machining time is 25 minutes and independent operator time is 1 minute per machine cycle. The operator costs $4 per hour and each machine costs $6 per hour.
(a) Determine the optimum number of machines to be assigned to the operator.
(b) Compute the cost per unit produced based on a three-machine assignment.
(c) If there are nine machines available for assignment and an operator is assigned sole responsibility for a specified number of machines, which assignment combination is preferred: (3, 3, 3) or (4, 5)? Base the choice on the average cost per unit produced.