Eastman Company purchased a delivery truck for $35,000 on January 1, 2008. 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 2008 and 22,000 miles in 2009. Compute depreciation expense using the units-of-activity method for the years 2008 and 2009.