Make a ventilation pipe with a length of 4 meters and a diameter of 20 meters. At least how many square meters of tinplate is needed?
用白铁皮做一根长4米的通风管，管口的直径是20米．至少需要白铁皮多少平方米？
<gadget id="calculator">3.14 * 20 * 4</gadget>
<output>251.2</output>

<result>251.2</result>