Answer:
AC = (20+ 10q)
Step-by-step explanation:
Given that,
Total cost, TC = 20q + 10q²
We need to find AC i.e. average cost.
It can be solved as follows :

So, the value of AC is (20+ 10q).
About 400 marbles. You simply set up a proportion and solve it as I did in the photo attached.
18 is D and 19 is C you're welcome