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. <em>If this isn't right I'd be happy to help with further information.</em>
So if we do not know what 12 times 9 is then we can round 9 to 10 and get 12 times 10 most people can multiply by ten so this should be easy so 12 times 10=120
12 times 9=108 she was off by 12 so a 10% deviation error
another way to do it is you multiply 12 by 10 and get 10 12's then you subtract 1 12 and get 9 12's