Your firm is in need of a machine that costs $5000. During the first year the maintenance costs are estimated to be $400. The maintenance costs are expected to increase by $150 each year up to a total of $1750 at the end of Year 10. The machine can be depreciated over 5 years using the straight-line method. Assume each year's depreciation is a known amount Dy. There is no salvage value. Develop an expression to show how you would find the most economical useful life of this machine on a before tax basis. Contributed by D. P. Loucks, Cornell University.