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?
The recipe calls for w cups of flour and x cups of sugar.

Mary has already put in y cups of flour.

Therefore, she needs to add w - y cups of flour and x - y cups of sugar.

The difference between the cups of sugar and cups of flour is: 

x - y - (w - y) = x - w

Therefore, the answer (symbolic expression) is: 

x - w