A wall 9 m long and 4 m high has one window measuring 2 m by 1 m. The wall is to be painted everywhere except the window, at ₹25 per square metre. What is the cost of the painting?

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 = $l\times b$.