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

Next, we need to find out how many cups of flour Mary has already put in. We know this is z cups. 

Finally, we need to subtract z from w + x to find out how many more cups of flour Mary needs to add. So Mary needs to add (w + x) - z cups of flour.