A machine that cost $84,000 has an estimated residual value of $21,000 and an estimated useful life of 21,000 machine hours. The company uses units-of-production depreciation and ran the machine 4,000 hours in year 1, 6,000 hours in year 2, and 8,000 hours in year 3 Calculate its book value at the end of year 3, (Do not round intermediate calculations.) Book Value ok nt ences