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 find out how many cups of flour the recipe calls for. We know that the recipe calls for w cups of flour and x cups of sugar. This means that the recipe calls for a total of w + x cups of ingredients. 

Now, we need to find out how many cups of flour Mary has already put in. We know that Mary has already put in y cups of flour. This means that she has put in a total of y + 0 cups of flour. 

Now, we need to find out how many more cups of sugar than cups of flour the recipe calls for. To do this, we need to subtract the amount of flour Mary has already put in from the amount of flour the recipe calls for. We know that the recipe calls for w + x cups of ingredients and Mary has already put in y + 0 cups of flour. This means that the recipe calls for (w + x) - (y + 0) cups of sugar. 

This means that the recipe calls for w - y more cups of sugar than cups of flour.

Therefore, the answer (symbolic expression) is: 

w - y