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?
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.

This means that she needs x - z cups of flour to complete the recipe.