Mary wants to decorate her Christmas tree. She wants to place the tree on a wooden box covered with coloured papers with pictures of Santa Claus. She must know the exact quantity of paper to buy for this purpose. If the box has length 80 cm, breadth 40 cm and height 20 cm respectively, then how many square sheets of paper of side 40 cm would she require?

By formula,

Surface area of the cuboid (box) = 2(lh + bh + hl)

= 2(80 × 40 + 40 × 20 + 20 × 80)

= 2(3200 + 800 + 1600)

= 2 × 5600

= 11200 cm².

Area of square sheet = (side)²

= 40²

= 1600 cm²

Let no. of sheets required to cover be n.

So, total area of square sheets = 1600n cm².

In order to cover the box with square sheets, their areas must be equal.

∴ 1600n = 11200

⇒ n = $\dfrac{11200}{1600}$ = 7.

Hence, 7 square sheets will be required.