fisher plastics uses an extruding machine in its manufacturing process. the machine costs $50,000, and annual after-tax operating expenses are $12,000 per year. If used for 4 years, it can be sold for an after-tax salvage value of $15,000. If used for 6 years the after tax salvage value would be zero. if the cost of capital is 15% should fisher use this machine for 4 or 6 years?