A company purchased a machine on January 1 of the current year for $750,000. Calculate the annual depreciation expense for each year of the machine's life (estimated at 5 years or 20,000 hours, with a salvage value of $75,000). During the machine's 5-year life its hourly usage was: 3,000; 4,000; 5,000; 5,000; and 3,000 hours. Calculate the depreciation expense for the five years using the straight-line method, units of production method and double-declining balance method.