The after-tax annual equivalent worth of retaining a defender over four years (physical life) or operating its challenger over six years (physical life) are as given in Table PI4.23.
TABLE PI4.23

If you need the service of either machine for only the next eight years, what is the best replacement strategy? Assume a MARR of 12% and no improvements in technology in future challengers.

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Defender $13.400 13,500 13,800 13,200 Challenger $12,300 13,000 13,600 13,400 13,000 12,500