Two 180-horsepower (HP) motors are being considered for installation at a municipal sewage treatment plant. Both motors are projected to have zero salvage value after a life of 10 years. If all the annual charges, such as insurance, maintenance, etc., amount to a 15% of the initial cost for each motor, and power costs are a flat five cents per kilowatt-hour. The MARR is equal to 6% per year. Note: A conversion factor you might find useful is 1 HP = 746 watts = 0.746 kilowatts. The first motor costs $5,800 and has an operating efficiency of 83%. The second motor costs $4,600 and has an efficiency of 80%. The minimum hours of full-load operation per year are necessary to justify purchasing the more expensive motor is equal to ...... hours/year