A three-year-old machine that has a cost of $51,100, an estimated residual value of $2,100, and an estimated useful life of 35,000 machine hours. The company uses units-of production depreciation and ran the machine 5,000 hours in year 1; 7,150 hours in year 2; and 8,400 hours in year 3.

Calculate the net book value at the end of the third year.