A bus company has a four-year-old natural gas powered bus that was initially purchased for $88000 This bus can be kept in service for an additional four years, or it can be sold for $25000 and replaced by a new one. The purchase price of the replacement machine is $75000. The projected MVs and operating and maintenance costs over the four year planning horizon are shown in the table that follows. Assuming the MARR is 15% (a) determine the economic life of the challenger: and (b) determine when (end of which year) the defender should be replaced Defender MV at EOY $22,000 $19,000 $10,000 $0 MV at EOY $60,000 $54,000 $48,000 $36,000 EOY O&M Cost $18,000 $25,000 $30,000 35,000 O&M Cost $10,000 $12,000 $14,000 $16,000 3