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?
First, we need to know how many cups of flour the recipe calls for in total. We can call this total number of cups of flour F. We know that the recipe calls for x cups of flour and we know that z cups of flour have already been added, so:

F = x + z

Now we need to know how many cups of flour Mary still needs to add. We can call this number f. We know that Mary has already added z cups of flour, so she still needs to add:

f = F - z

f = (x + z) - z

f = x