Water poured into a rectangular tank takes the shape of a cuboid, so nothing new is needed: `volume of water = length × width × depth of the water`. The two traps are both about reading. First, a part-filled tank gives you two vertical measurements — the tank's height and the water's depth — and only the water's depth belongs in the calculation. Second, the answer is usually wanted in litres: `1 ℓ = 1000 cm³`, so you **divide** cm³ by 1000 to get ℓ and multiply the other way. A sanity check that never fails: the number of litres must be smaller than the number of cm³.
Worked example
Problem A rectangular tank measures 40 cm by 20 cm by 30 cm. It contains water to a depth of 15 cm. Find the volume of water in ℓ, and the volume still needed to fill the tank.