A cylindrical ventilation pipe with a base radius of 5 cm and a length of 4 decimeters. At least how many square meters of iron sheets are needed to make 200 such ventilation pipes.
一种圆柱形通风管，底面半径是5厘米，长4分米．做200根这样的通风管至少需要铁皮多少平方米．
<gadget id="calculator">5 / 100</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">4 / 10</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">2 * 3.14 * (1/20) * (2/5) * 200</gadget>
<output>25.12</output>

<result>25.12</result>