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.