Two 100-horsepower (HP) motors are being considered for installation at a municipal sewage-treatment plant. The first costs $1,900 and has an operating efficiency of 80%. The second costs $6,200 and has an efficiency of 90%. Both motors are projected to have zero salvage value after a life of 10 years. If the annual charge for the first motor is $170 and for the second is 310, and if power costs are a 0.10 per kilowatt-hour, how many minimum hours of full-load operation per year are necessary to justify purchasing the more expensive motor at i = 12%? (A conversion factor you might find useful is 1 HP = 746 watts = 0.746 kilowatts.)