In an aquarium 50 cm long and 20 cm wide, water is poured up to a height of 25 cm.

a = 50 cm = 0.5 m.

b = 20 cm = 0.2 m.

h = 25 cm = 0.25 m.

ρ = 1000 kg / m3.

m -?

The density of a substance ρ is a scalar physical quantity equal to the ratio of the mass of a substance m to the volume V it occupies: ρ = m / V.

We express the mass of water m by the formula: m = ρ * V.

Since the aquarium has the shape of a rectangular parallelepiped with edges a * b * h, we will express its volume V by the formula: V = a * b * h.

m = ρ * a * b * h.

m = 1000 kg / m3 * 0.5 m * 0.2 m * 0.25 m = 25 kg.

Answer: the aquarium contains m = 25 kg of water.