18. Business: Demand and Revenue Demand for an electric fan is related to its sell- ing price p (in dollars) by the equation n = 2,880 - 90p where n is the number of fans that can be sold per month at a price p. Find the selling price that will maximize the revenue received. 21. Business: Maximum Revenue A tool company determines that it can achieve 500 daily rentals of jackhammers per year at a daily rental fee of $30. For each Sl increase in rental price, 10 fewer jackhammers will be rented. What rental price maximizes revenue