Semiautomatic machines are used to produce a particular product. It takes 4 minutes to load and 3 minutes to unload a machine. A machine runs automatically for 25 minutes in producing one unit of the product. Travel time between machines is 20 seconds. While machines are automatically running, the operator inspects the unit previously produced; 75 seconds are required to inspect one unit. An operator costs $15 per hour, and a machine costs $40 per hour. Determine the number of machines assigned to an operator to minimize the cost per unit produced.