Mary is baking a cake. The recipe calls for 9 cups of flour and 11 cups of sugar. She already put in 4 cups of flour. How many more cups of sugar than cups of flour does she need to add now?
Let's denote the number of cups of flour that Mary has already put in by x. Then the number of cups of sugar that she has already put in is y. 

The recipe calls for 9 cups of flour and 11 cups of sugar. Therefore x+y=9 and x+y=11. 

From the first equation we get y=9-x. Substituting this into the second equation we get: 

9-x+y=11

9-x+9-x=11

9-2x=11

-2x=-2

x=1/2

y=9-1/2=8 1/2

Therefore Mary needs to add 8 1/2 cups of sugar.