Two 180-horsepower (HP) motors are being considered for installation at a municipal sewage-treatment plant. The first costs $4, 800 and has an operating efficiency of 84%. The second costs $4, 000 and has an efficiency of 81%. 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 total of 18% of the original cost of each motor, and if power costs are a flat five cents per kilowatt-hour, how many minimum hours of full-load operation per year are necessary to justify purchasing the more expensive motor at i= 6%? (A conversion factor you might find useful is 1 HP = 746 watts = 0.746 kilowatts.)