Every x and y coordinate is different. The y-coordinates don't have the same x-coordinate, so the function passes the vertical line test. This function is called a discrete function since it's made up of individual points, not a line or curve.
Since Carmen gives two coins to Frankie for every five that she gets, she effectively only gains 3 coins at a time. Therefore, if she has 9 coins, Frankie must have 6.
