9-5. 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 three 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, and MARR is 10% per year. What is the economic life of this challenger? (9.5)