The ventilation pipe only needs the side area, because both ends are used for ventilation.

3. 14×(5×2 )×80

=3. 14× 10×80

=3. 14×800

=25 12 (square centimeter)

25 12×20=50240 (square centimeter)

A: It takes at least 50,240 square centimeters of iron sheet to make 20 such ventilation pipes.