Problem $7(10$ points$).$ Last year Machine A was purchased for $$15,000.$ Machine A has a useful life of five years and four years remain. However, Machine A has not performed as expected and annual operating costs have been $$9000$ per year. An alternative, Machine B can be purchased for $$25,000$ and is expected to have an annual operating cost of $$2500$ per year. The expected useful life of Machine $B$ is five years and the salvage of both machines is negligible. Assuming a minimum rate of return of $6\%,$ when should you purchase Machine B to replace Machine A$?$