In a replacement analysis for an industrial saw, the following data are known about the challenger. Initial investment is $18,000. Annual maintenance costs begin at the end of year three, with a cost at that time of $1,000, with $1,000 at the end of year four, increasing by $8,600 each year thereafter. The salvage value is $0 at all times. Using a MARR of 6% per year, what is the economic life of the challenger?