I take it that you have access to an animation.
For every 4 plates, you want to put 6 cookies and 2 brownies (in any combination I think) do this twice to get to 8 plates and then use 3 cookies and 1 brownie for the last 2 plates.
Again, I am not sure about this as I don't have access to the animation and not certain I am coming from the right angle. I think the purpose is to show that 15 cookies and 5 brownies are required.
If this isn't right I'd be happy to help with further information.