circular iron ring has a diameter of 40 cm. How long is the iron bar needed to make such an iron ring?
一个圆形的铁环，直径是40厘米，做这样一个铁环需要用多长的铁条？
<gadget id="calculator">3.14 * 40</gadget>
<output>125.6</output>

<result>125.6</result>