$9-5.$ In a replacement analysis for a vacuum seal ona spacecraft, the following data are known about thechallenger: the initial investment is $$12,000$; there is noannual maintenance cost for the first three years, however,it will be $$2,000$ in each of years four and five, and then$$4,500$ in the sixth year and increasing by $$2,500$ eachyear thereafter. The salvage value is $$0$ at all times, andMARR is $10\%$ per year. What is the economic life of thischallenger? $(9\mathrm{.}5)$