A machine with a useful life of 3 years is purchased for $100,000. It is known that the market value of the machine decreases by 10% each year. 'The book value can be calculated by assuming straight-line depreciation over a 4-year depreciable life to a salvage value of $20,000. The machine generates a gross income of $50,000 annually. The operating and maintenance costs are $30,000 in year 1, increasing by $5,000 each year. Determine the after-tax marginal costs if the tax rate is 40% and 10%. If the minimum EUAC of a challenger is $56,000 when should the machine be replaced?