On January 1, 2020, Walker Inc. acquired equipment for $72,000. The expected useful life is 10 years and the residual value is $1,440. Total service hours for the equipment are estimated to be 18,000 while actual hours for 2020 were 1,620. Compute depreciation expense in 2020, under the following methods. Note: Round your answers to the nearest whole dollar. a. Straight line b. Sum-of-the-years-digits $ Double-declining-balance $ d. Units-of-production