Helicopter Gear is planning to expand its product line, which requires investment of $475,200 in special-purpose machinery. The machinery has a useful life of six years and no salvage value. The estimated annual results of offering the new products are as follows: Revenue Expenses (including straight-line depreciation) Increase in net income $ 528,000 (501,600) $ 26,400 All revenue from the new products and all expenses (except depreciation) will be received or paid in cash in the same period as recognized for accounting purposes. The payback period for this proposed investment is