A three-year-old machine has a cost of $22,500, an estimated residual value of $2,500, and an estimated uscful life of 40,000 machine hours. The company uses units-of-production depreciation and ran the machine 3,800 hours in year 1,7,650 hours in year 2 ;. and 7,300 hours in year 3 . Calculate the net book value at the end of the third year