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?

We are looking for the number of cups of flour that Mary needs to add. We are given that she has already put in z cups of flour. So we want to know w - z. 

We are also told that the recipe calls for w cups of sugar, x cups of flour, and y cups of salt. So w + x + y = the total number of cups in the recipe. 

We can set up an equation and solve for x: 

w + x + y = the total number of cups in the recipe

x = the total number of cups in the recipe - w - y

x = the total number of cups in the recipe - (w + y)

Now we have everything we need to solve for w - z: 

w - z = the total number of cups in the recipe - (w + y) - z

w - z = the total number of cups in the recipe - w - y - z

w - z = the total number of cups in the recipe - w - y - z

w - z = the total number of cups in the recipe - (w + y + z)

w - z