William has a 26-liter glass tank. First, he wants to put some marbles in it, all of the same volume. Then, he wants to fill the tank with some water until its comepletly full. If he uses 85 marbles, he will have to add 20.9 liters of water. What is the volume of each marble? and How much water is necessary if William uses 200 Marbles?

Total capacity = 26 liters

He added in 20.9 liters of water, so if we take out the water …
26 – 20.9 = 5.1 liters
The 5.1 liters volume was made up by the 85 marbles

85 marbles = 5.1 liters
1 marble = 5.1 ÷ 85
1 marble = 0.06 liter

If he uses 200 marbles
200 x 0.06 = 12 liters
The 200 marbles will take up 12 liters

Amount of water needed = 26 - 12
Amount of water needed = 14 liters