A cylindrical ventilation pipe with a base radius of 5 cm and a length of 8 decimeters. How many square meters of iron sheets are needed to make 20 such ventilating pipes?
一种圆柱形通风管，底面半径是5厘米，长8分米．做20根这样的通风管至少需要铁皮多少平方米？
<gadget id="calculator">5 / 10</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">3.14 * (1/2) * 2 * 8 * (1/5)</gadget>
<output>5.024</output>

<result>5.024</result>