A three-year-old machine has a cost of $26,900, an estimated residual value of $2,900, and an estimated useful life of 40,000 machine hours. The company uses units-of-production depreciation and ran the machine 4,700 hours in year 1; 6,250 hours in year 2; and 8,000 hours in year 3. Calculate the net book value at the end of the third year