The sum of the diameters of two circles is 30 cm, the radius of one of them is r cm, and the circumference of the other circle.
两个圆的直径之和为30厘米，其中一个圆的半径为r厘米，另一个圆的周长为多少．
<gadget id="calculator">3.14 * 30</gadget>
<output>94.2</output>

<gadget id="calculator">3.14 * 2</gadget>
<output>6.28</output>

<gadget id="calculator">94.2 - 6.28</gadget>
<output>87.92</output>

<result>87.92</result>