Whippet bus lines uses the units of activity method in depreciating it's buses one bus was purchased on January 1,2017 at a cost of $105,000 over its 4 years useful life the bus is expected to be driven 190,000 miles salvage value is expected to be $8,100 so what is the depreciation cost per unit and could you answer the years also 2017/2018/2019/2020