To make 20 ferrules, the radius is 4 cm, and 2 cm should be left at the knot, how many centimeters are needed.
要做20个套圈，半径为4厘米，结头处要留2厘米，需要多少厘米．
<gadget id="calculator">2 * 3.14 * 4</gadget>
<output>25.12</output>

<gadget id="calculator">25.12 + 2</gadget>
<output>27.12</output>

<gadget id="calculator">27.12 * 20</gadget>
<output>542.4</output>

<result>542.4</result>