Mary is baking a cake. The recipe calls for 11 cups of sugar and 9 cups of flour. She already put in 12 cups of flour and 10 cups of sugar. How many more cups of sugar does she need to add?
We know that the recipe calls for 11 cups of sugar and 9 cups of flour. 

We know that Mary has already put in 12 cups of flour and 10 cups of sugar. 

We want to know how many more cups of sugar Mary needs to add. 

We can solve this problem by subtracting what Mary has already put in from what the recipe calls for. 

11 cups of sugar - 10 cups of sugar = 1 cup of sugar

9 cups of flour - 12 cups of flour = -3 cups of flour

This means that Mary needs to add 1 more cup of sugar, but she has 3 cups of flour too many.