Compute the payback period for each of these two separate investments:
a. A new operating system for an existing machine is expected to cost $250,000 and have a useful life of four years. The system yields an incremental after-tax income of $72,115 each year after deducting its straight-line depreciation. The predicted salvage value of the system is $10,000.
b. A machine costs $180,000, has a $13,000 salvage value, is expected to last seven years, and will generate an after-tax income of $39,000 per year after straight-line depreciation.