A floor measures 4.8 m by 3 m. It is to be covered completely with square tiles of side 20 cm, laid with no gaps and no tile cut. How many tiles are needed?

Work in centimetres: the floor is 480 cm by 300 cm, so its area = $144000{\text{cm}}^2$, and each tile covers $20\times 20=400{\text{cm}}^2$. Number of tiles = 144000/400 = 360. Counting by rows agrees: 480/20 = 24 tiles along one side and 300/20 = 15 along the other, giving $24\times 15=360$. (Reading $1{\text{m}}^2$ as $1000{\text{cm}}^2$ instead of $10000{\text{cm}}^2$ would wrongly give 36.)