In a replacement analysis for a vacuum seal on a spacecraft the following data are known about the challenger. The initial investment is $12,000 , there is no annual maintenance cost for the first 3 years , however , it will be $2,000 in each of years four and five , and then $4,500 in the sixth year and increasing by $2,500 each year thereafter . The salvage value is $0 at all times , the MARR is per year. What is the economic life of this challenger?