How many square meters of iron sheets are needed to make 20 cylindrical ventilation pipes with a section diameter of 40 cm and a length of 6 decimeters?
制作20节截面直径40厘米，长6分米的圆柱形通风管，需要铁皮多少平方米？
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">6 / 10</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">3.14 * (2/5) * (3/5) * 20</gadget>
<output>15.072</output>

<result>15.072</result>