Ritu has a rectangular chart paper 30 cm long and 18 cm wide. She folds it exactly in half so that it becomes a 15 cm by 18 cm rectangle. The area is now half of what it was — what has happened to the perimeter?

Area measures how much flat space a shape covers. The unit is a **square** of side 1 — so ${\text{cm}}^2$ (square centimetres), $m^2$ (square metres), etc. To measure the area of a rectangle visually, imagine covering it with 1-cm squares: count them. A rectangle 5 cm by 3 cm holds $5\times 3=15$ little squares, so its area is $15{\text{cm}}^2$. The general formula is the same idea: area of rectangle = $l\times b$.