To make a chimney with a bottom diameter of 20 cm and a length of 90 cm, at least how many square decimeters of iron sheet are needed.
做一节底面直径为20厘米，长90厘米的烟筒，至少需要多少平方分米铁片．
<gadget id="calculator">1 * 100</gadget>
<output>100</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">3.14 * 20 * (9/10)</gadget>
<output>56.52</output>

<result>56.52</result>