An asset costs $20,000 and is in the 5 year property class. O&M costs are $2,000 the first year, $3,000 the second year, $4,000 the third year, etc. Salvage values are $12,000 after one year, $9,000 after 2 years, $6,000 after 3 years, and $4,000 thereafter. Depreciation is by SL, without the half year convention and MARR = 10%; the marginal tax rate is 40%. Use the standard SL depreciation approach to find the after tax economic life.