At Dorben Company, a worker is assigned to operate several machines. Each of these machines is down at random times during the day. A work sampling study indicated that on the average the machines operate 60 percent of the time unattended. Operator attention time at irregular intervals averages 40 percent. If the machine rate is $20 per hour and the operator rate is $12 per hour, what would be the most favorable number of machines (from an economic standpoint) that should be operated by one operator?