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 find out how many cups of flour the recipe calls for in total. We can do this by adding w and x:

total_flour = w + x

Now we need to find out how much flour Mary has already put in. We know this is z cups:

flour_added = z

To find out how much flour Mary still needs to add, we need to subtract the flour she has already put in from the total amount of flour the recipe calls for:

flour_needed = total_flour - flour_added

Now we have all the information we need to answer the question. We just need to substitute the values into our equation and simplify:

flour_needed = (w + x) - z

flour_needed = w + x - z