able towing company purchased a tow truck for $180,000 on january 1, 2017. it was originally depreciated on a straight-line basis over 10 years with an assumed salvage value of $36,000. on december 31, 2019, before adjusting entries had been made, the company decided to change the remaining estimated life to 4 years (including 2019) and the salvage value to $5,000. what was the depreciation expense for 2019?