6 L of water was poured into the aquarium, and it turned out that the aquarium was one third full.

6 L of water was poured into the aquarium, and it turned out that the aquarium was one third full. How many liters of water can the aquarium hold?

The aquarium is three thirds (3/3). It turns out that this aquarium holds 18 liters of water, since one third of the aquarium holds 6 liters of water, therefore the whole aquarium is three of the same parts: 3 * 6 = 18. Or you can solve it by adding: 1/3 + 1/3 + 1/3 = 6 + 6 + 6.