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 sugar she needs to add in total. We can call this variable z. 

We know that she needs w cups of flour and x cups of sugar in total. We also know that she has already put in y cups of flour. 

This means that she needs to put in w-y cups of flour and x cups of sugar in total. 

We also know that she has already put in y cups of flour, which means that she needs to put in z-y cups of sugar. 

We can set these two equations equal to each other and solve for z. 

w-y=z-x 
z=w-y+x