Computing Depreciation Expense for Change in Useful Life Phelps Company purchases equipment on January 1, 2019. for $21,600 which will be depreciated using the sum-of-years-digits method. The equipment has a residual value of $3,600 and a useful life of three years. In 2020, Phelps decides that the machine has an original total useful life of four years and $1,800 salvage value, Wazis depreciation expense for 20202 Depreciation expense for 2020:50 Check