Mary is baking a cake. The recipe calls for w cups of sugar and x cups of flour. She already put in y cups of flour and z cups of sugar. How many more cups of sugar does she need to add?
First, we need to figure out how many cups of sugar she has put in so far. We know that she put in z cups of sugar, and the recipe calls for w cups of sugar. So we can say that:

z + s = w

where s is the number of cups of sugar she still needs to put in. 

We also know that she put in y cups of flour, and the recipe calls for x cups of flour. So we can say that:

y + f = x

where f is the number of cups of flour she still needs to put in. 

We can solve for s and f:

s = w - z
f = x - y

Now we need to figure out how many cups of sugar she needs to add. We know that she put in z cups of sugar, and she needs to add s cups of sugar. So we can say that:

z + a = s

where a is the number of cups of sugar she needs to add. 

We can solve for a:

a = s - z