a linear algebraic formulation model

ABC is selling 3-foot, 5-foot, and 9-foot bar from steel only on special order basis – and all lengths of boards are the same width and thickness. One of ABC's customers, has placed an order for 25 3-foot long bars, 20 5-foot long bars, and 15 9-foot long bars. ABC meets all its customers’ demands by cutting up 17-foot long bar. How can Woodco satisfy these demand – that is how many bar must be cut up – with the least possible amount of waste from the cut bar ?