A three-year-old machine has a cost of $47,100, an estimated residual value of $2,300, and an estimated useful life of 32,000 machine hours. The company uses units-of-production depreciation and ran the machine 4,800 hours in year 1; 7,250 hours in year 2; and 7,600 hours in year 3 . Calculate the net book value at the end of the third year