You might think that the volume of a circle is needed to solve this problem but you can do it in a more simplified manner. First figure out how much water 85 marbles displace.
26-20.9=5.1 liters of water
Now, let us see how much water 200 marbles displace.
85 marble x (200/85) = 5.1 liters x (200/85)
200 marbles = 12 liters
We can find the amount of water needed to fill the tank now.
26-12=14 liters of water
answer: 14 liters of water
