Answer:
602.88 cu ft.
Explanation:
V = Bh where B is the area of the base, which is a circle in your problem.
Since the diameter is 8, the radius is 4.
So, B =
V = Bh = 50.24(20) = 1004.8 cu ft = total volume of tank
60% of that volume is .60(1004.8) = 602.88 cu ft.