An asset, purchased three years ago for $20,000, can be replaced by a new type of equipment. The market value of the old machine is currently $13,000 and will be $9,000, $8,000, $6,000, $2,000, and $0 at the end of each of the next five years. The annual operating costs for each of the next five years will be $2,500, $2,700, $3,000, $3,500, and $4,500. How much longer should the defender be kept, if MARR is 10%?