How can I calculate NPV using depreciation? What is the formula for calculating cash flows with depreciation?

Your firm, Agrico Products, is considering a tractor that would have a net cost of $36,000, would increase pre-tax operating cash flows (i.e., sales - operating costs excluding depreciation) before taking account of depreciation by $12,000 per year, and would be depreciated on a straight-line basis to zero over 5 years at the rate of $7,200 per year, beginningthe first year.(Thus annual cash flows would be $12,000, before taxes, plus the tax savings that result from $7,200 of depreciation.) The managers are having a heated debate about whether the tractor would actually last 5 years.The controller insists that she knows of tractors that have lasted only 4 years.The treasurer agrees with the controller, but he argues that most tractors actually do give 5 years of service.The service manager then states that some actually last for as long as 8 years.

Given this discussion, the CFO asks you to prepare a scenario analysis to determine the importance of the tractor's life on NPV.Use a 40 percent marginal federal-plus-state tax rate, a zero salvage value, and a WACC of 10 percent. Assuming each of the indicated lives has the same probability of occurring (probability = 1/3), what is the tractor's expected NPV?