Computing Book Value (Units-of-Production Depreciation) - Calculate the book value of a three-year-old machine that has a cost of $21,000, has an estimated residual value of $1,000 and an estimated useful life of 40,000 machine hours. The company uses units-of production depreciation and ran the machine 3,200 hours in year 1; 7,050 hours in year 2; and 7,500 hours in year 3.