Kingbird Company purchased a delivery truck for $40,000 on January 1,2020 . The truck was assigned an estimated useful life of 100,000 miles and has a residual value of $10,000. The truck was driven 18,000 miles in 2020 and 22,000 miles in 2021 . Compute depreciation expense using the units-of-activity method for the years 2020 and 2021. Depreciation expense for 2020 $ Depreciation expense for 2021 $