Griffin Dewatering purchases a wellpoint pump connected to a skid-mounted diesel engine for \($14,000.\) These are considered ‘‘construction assets’’ and are classified as MACRS-GDS 5-year property (remember the half-year convention). Its market value for salvage purposes decreases by 30 percent each year. When installed on a construction job, a wellpoint system operates virtually 24/7, and operating and maintenance costs will be \($3,500\) the first year, increasing by \($600\) each year thereafter. Griffin is in the 40 percent tax bracket, and their after-tax MARR is 9 percent. Determine the after-tax optimum replacement interval. Do not consider a Section 1031 exchange. If the salvage value differs from the book value when replacement occurs, the difference is taxed at the normal income tax rate.