How many square pieces of cardboard of side 40 cm are required to make a box of dimensions 80 cm × 40 cm × 20 cm?

7

Length =80 cm, Breadth = 40 cm, Height = 20 cm.

The surface area of the box = 2(lb + bh + hl)

= 2[(80 × 40) + (40 × 20) + (20 × 80)] cm²

= 2[3200 + 800 + 1600] cm²

= 2 × 5600 cm2 = 11200 cm²

The area of each sheet of the paper = 40 × 40 cm2 = 1600 cm²

Therefore, number of sheets required = surface area of box / area of one sheet of paper = 11200/ 1600 = 7