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?
We know:
4 cups of flour -> x cups of sugar

We need to find:
x cups of sugar -> y cups of sugar

We are looking for:
y - x cups of sugar

Now we can write down the equation:
9x = 11y
y = (9x)/11