16•2/5=6.4
6.4 ounces of chocolate for the frosting.
16-6.4=9.6
9.6 ounces of chocolate for the brownies.
9.6-6.4=3.2
3.2 ounces of chocolate for the brownies more than from the frosting.
(I hope I understood your question right. I don't have good English)