There are two identical vessels, X and Y. Y is filled with water to the brim and X is empty. There are two pails A and B, such that B can hold half as much water as A. One operation is said to be executed when water is transferred from Y to X using A once and water is transferred to Y from X using B once. If A can hold a liter of water and it takes 40 operations to equate the water level in X and Y, what is the total volume of water in the system?

• Answer: Option C.
  In one complete operation, water transferred = 1 - (1/2) = 1/2 liters as 1 liter goes from Y to X. So 1 liter is contained in X. But ½ liter goes from X to Y as well. So net ½ liters remain in X. If it takes 40 operations to equate water level,
  ∴ 40 × (1/2) = 20 liters is contained in X and Y.
  Hence total volume of water in the system is 40 litres. Hence, 3rd option.
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