The annual equivalent after-tax costs of retaining a defender machine over 4 years (physical life), or operating its challenger over 6 years (physical life), are as follows: n Defender Challenger 1 -$3,200 -$5,800 2 -2,500 -4,230 3 -2,650 -3,200 4 -3,300 -3,500 5 -4,000 6 -5,500 If you need the service of either machine for only the next 10 years, what is the best replacement strategy? Assume a MARR of 12% and no technology improvement in future challengers.