Mary is baking a cake. The recipe calls for w cups of sugar and x cups of flour. She already put in y cups of flour and z cups of sugar. How many more cups of sugar does she need to add?
First, we need to figure out how many cups of sugar are in the recipe. We can do this by adding w + z. 

Next, we need to figure out how many cups of flour are in the recipe. We can do this by adding x + y. 

Finally, we need to figure out how many more cups of sugar Mary needs to add. We can do this by subtracting y from w.