A three-year-old machine has a cost of $30,200, an estimated residual value of $2,200, and an estimated useful life of 40,000 machine hours. The company uses units-of-production depreciation and ran the machine 4,800 hours in year 1; 7,450 hours in year 2; and 8,400 hours in year 3. Calculate the net book value at the end of the third year.