A three-year-old machine that has a cost of $24,500, an estimated residual value of $1,400, and an estimated useful life of 33,000 machine hours. The company uses units-of production depreciation and ran the machine 3,500 hours in year 1; 6,950 hours in year 2; and 8,200 hours in year 3. Calculate the net book value at the end of the third year.