Griffin Dewatering purchases a wellpoint pump connected to a skid-mounted diesel engine for \($14,000. Its\) market value for salvage purposes decrease 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. What is the optimum replacement interval if MARR ¼ 15 percent?