Tank A contains 10 liters of water. If two-fifths of this water is poured into container B

Tank A contains 10 liters of water. If two-fifths of this water is poured into container B, then the amount of water in both containers will become equal. How much water was in tank B initially.

It is known that two-fifths of water from 10 liters was poured into container B. Let’s calculate how much water was poured.
10 liters – is 5 parts;
x liters – is 2 parts.
Hence, 10 ÷ 5 × 2 = 4 liters.
This means that 4 liters have been poured.
10 – 4 = 6 liters.
That is, 6 liters remained in tank A, and 6 liters became in tank B, since they became equal.
We calculate how many liters of water were in tank B. To do this, we subtract from the amount of water that has become in the tank, the amount of water that was added from tank A, we get:
6 – 4 = 2 liters.
Answer: Tank B contained 2 liters of water.