Hadicke Company purchased a delivery truck for $45,000 on January 1, 2012. 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 2012 and 22,000 miles in 2013. Compute depreciation expense using the units-of-activity method for the years 2012 and 2013.