Assuming you want you answer is ounces -

2/5 of 16 (the amount of chocolate she used in frosting) is 6.4. So she used 6.4 ounces in the frosting.

16 - 6.4 (the amount she used in the brownies) is 9.6. We are now trying to find the difference of these two numbers, which is 3.2

So 3.2 is the answer.

Hope this helped!