If you add 15 more cups to the 5 cups of apple juice, you would have 20 cups, and so I'm assuming you want to find how much cups of grape juice do you have. The ratio of grape juice to apple juice is 3:5. If you multiply the base, 5 cups of apple juice time 4, you get 20 cups of apple juice. So, all you need to do is multiply the 3 cups of grape juice by 4 also. And that would get you 12 cups. So when there is 20 cups of apple juice, there is 12 cups of grape juice.