Compute the payback for each of these two seperate investments:

a. A new operating system for an existing machine is expected to cost $250000 and have a useful life of 6 years. The system yields an incremental after-tax income of $72115 each year after deducting its straight line depreciation. The predicted salvage value of the system is $10000.

b. A machine costs $200,000, has a $13,000 salvage value, is expected to last eight years, and will generate an after-tax income of $39,000 per year after straight-line depreciation.