These are equations. San Francisco has an order for 600 books. They have to get exactly 600 books. More or less will not do. If they get less than 600 books, students will be going without texts, but if they get more than 600 texts, they will say, "We only orderd 600 texts. We're not paying for any more." Similarly, Sacramento needs exactly 400 texts. As a result, we can solve these equations to express z and w in terms of x and y.