A three-year-old machine has a cost of $24,000, an estimated residual value of $3,000, and an estimated useful life of 35,000 machine hours. The company uses units-of-production depreciation and ran the machine 5,100 hours in year 1: 7,850 hours in year 2; and 7800 hours in year 3. Calculate the net book value at the end of the third year. Net book value