Helix Company purchased tool sharpening equipment on April 1, 2010, for $72,000. The equipment was expected to have a useful life of three years, or 9,000 operating hours, and a residual value of $2,700. The equipment was used for 2,400 hours during 2010, 4,000 hours in 2011, 2,000 hours in 2012, and 600 hours in 2013.

Instructions
Determine the amount of depreciation expense for the years ended December 31, 2010, 2011, 2012, and 2013, by
(a) The straight-line method,
(b) The units-of-production method, and
(c) The double-declining-balance method.