3. prefab, a furniture manufacturer, uses 20,000 square feet of plywood per month. its trucking company charges prefab $400 per shipment, independent of the quantity purchased. the manufacturer offers an all unit quantity discount with a price of $1 per square foot for orders under 20,000 square feet, $0.98 per square foot for orders between 20,000 square feet and 40,000 square feet, and $0.96 per square foot for orders larger than 40,000 square feet. prefab incurs a holding cost of 20 percent. (28 points total)