Roberto needs 6 cookies and 2 brownies for every 4 plates he makes for a bake sale. drag cookies and brownies into the box to show how many roberto needs for 10 plates.

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.

For every 4 plates he needs 6 cookies and 2 brownies. Lets find out how many he needs per plate. To do this we will divide. Lets do it:- 

Cookies:-
6 ÷ 4 = 1.5
1.5 (1 and a half) cookie per plate.

CHECK OUR WORK:-

1.5 × 4 = 6
We were RIGHT!!

Brownies:-

2 ÷ 4 = 0.5
0.5 (half) brownie in each plate.

CHECK OUR WORK:-

0.5 × 2 = 4
We were RIGHT!!!

Now lets multiply each item by 10 to find out how many f each item he will need for 10 plates.

Cookies:-

1.5 × 10 = 15
For 10 plates he needs 15 cookies.

Brownies:-

0.5 × 10 =  5
For 10 plates he needs 5 brownies.

So, Roberto needs 15 cookies and 5 brownies for 10 plates.

Hope I helped ya!! xD