Mary is baking a cake. The recipe calls for 3 cups of sugar 10 cups of flour and 15 cups of salt. She already put in 6 cups of flour. How many more cups of flour does she need to add?
First, we need to realize that the recipe is for a very large cake. It calls for 3 cups of sugar, 10 cups of flour, and 15 cups of salt. 

Second, we need to realize that Mary has already put in 6 cups of flour. 

That means that she needs to add 4 more cups of flour to the cake batter.