Omaha Beef Co. purchased a delivery truck for $50,000. The residual value at the end of an estimated eight-year service life is expected to be $10,000. The company uses straight-line depreciation for the first six years. In the seventh year, the company now believes the truck will be useful for a total of 10 years (four more years), and the residual value will remain at $10,000. Calculate depreciation expense for the seventh year.