on december 31, 2021, brown co. had a machine with an original cost of $90,000, accumulated depreciation of $75,000, and an estimated salvage value of zero. on december 31, 2021, brown was considering the purchase of a new machine having a five-year life, costing $150,000, and having an estimated salvage value of $30,000 at the end of five years. in its decision concerning the possible purchase of the machine, how much should brown consider as sunk cost at december 31, 2021?