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?
Let's denote the number of cups of flour Mary needs to add by the variable x. 

We know that Mary has already put in 6 cups of flour. 

This means that the total number of cups of flour in the cake batter is: 

3 cups of sugar + 10 cups of flour + 15 cups of salt + 6 cups of flour = 3 + 10 + 15 + 6 = 34 

We know that the recipe calls for a total of 3 + 10 = 13 cups of flour. 

So we can write: 

3 + 10 + x = 34 

10 + x = 34 

x = 24 

Mary needs to add 24 more cups of flour to the cake batter.