A 450 piece/hr 10 hr B 500 piece/hr 8 hr A set of unsharpen ed tools costs $1,500 and can be ground 15 times. The cost of each grinding is $20. The time required to change and reset the tools is 2 hours, and such changes are made by a tool-setter who is paid $16/hour. The production machine operator is paid $15/hour, including the time that the machine is down for tool sharpening. Variable overhead on the machine is charged at the rate of $30/hour, including tool-changing time. At what speed should the machine be operated to minimize the total cost per piece?