A cuboid of plasticine has sides 5 cm, 2 cm and 5 cm. The MINIMUM number of such cuboids needed to form a perfect cube (without leftover) is:

Volume of one cuboid = 5 × 2 × 5 = 50 cm³. To form a cube, we need a cube of some side a so a³ must be a multiple of 50. The smallest cube made from these is one where each side is the LCM of (5, 2, 5) = 10 cm. Volume = 10³ = 1000 cm³. Number of cuboids = 1000 / 50 = 20.