A three-year-old machine has a cost of $27,100, an estimated residual value of $2,300, and an estimated useful life of 31,000 machine hours. The company uses units-of-production depreciation and ran the machine 4,800 hours in year 1; 6,150 hours in year 2; and 7,800 hours in year 3. Calculate the net book value at the end of the third year. Net book value $