company F is currently buying a part at a cost of $12 each. it is considering buying a machine that will produce the part at a variable cost of $8. each unit of input produces the part plus a by-product, which is sold for $1. the machine will cost $40.000 and will have a useful life of 5 years. the firm requires an 8% return. what annual volume is necessary to justify making the investment? ignore income taxes