4. A snow loading machine costing $30,000 requires four operators at $36 per day. The machine can do the work of 50 hand shovelers at $21 per day. Fuel, oil and maintenance for the machine amount to $60 per day. Assume the life of the machine is 8 years with no salvage value. If interest on money is 6%, how many days of snow removal per year are necessary to make purchases of the machine economical