A publisher needs to send many books to a local book retailer and will send the books in a combination of small and large boxes. Each small box can hold 20 books and each large box can hold 40 books. There were 4 times as many large boxes sent as small boxes, which altogether can hold 360 books. Graphically solve a system of equations in order to determine the number of small boxes sent, x, and the number of large boxes sent, y.