To make 20 identical cylindrical ventilation pipes, the bottom area of the ventilation pipe has a diameter of 20 cm and a length of 2 meters. At least how many square meters of iron sheets are needed to make these ventilation pipes?
要做20节相同的圆柱形通风管，通风管的底面积直径是20厘米，长是2米．做这些通风管至少需要多少平方米的铁皮？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">3.14 * (1/5) * 2 * 20</gadget>
<output>25.12</output>

<result>25.12</result>