Mary is baking a cake. The recipe calls for w cups of flour and x cups of sugar. She already put in y cups of flour. How many more cups of sugar than cups of flour does she need to add now?
First, we need to figure out how many cups of flour she needs in total. The recipe calls for w cups of flour, and she already put in y cups of flour. So she needs w - y cups of flour in total. 

Next, we need to figure out how many cups of sugar she needs in total. The recipe calls for x cups of sugar. 

Now we need to compare the two. The recipe calls for w - y cups of flour and x cups of sugar. We want to know how many more cups of sugar than cups of flour she needs to add. So we take x - (w - y) = x - w + y.