The diameter of the bottom surface of a cylindrical ventilation pipe is 20 cm, and the length is 40 cm. To make 20 such ventilation pipes, how many square centimeters of iron sheet should be used at least?
一个圆柱形通风管的底面直径是20厘米，长是40厘米，做20根这样的通风管，至少要用多少平方厘米的铁皮？
<gadget id="calculator">3.14 * 20 * 40 * 20</gadget>
<output>50_240</output>

<result>50_240</result>