He would need 15 cookies and 5 brownies. This is because 4 is not a multiple of 10, so we are going to reduce it to 2. This means that you need 3 cookies and 1 browny for every 2 plates. 10 divided by 2 is 5, so you multiply each treat by 5 and get 15 cookies and 5 brownies. Hope this helps!!