Cut the largest circle out of a square piece of paper with a circumference of 40 cm. What is the radius of the circle in cm?
在一个周长是40cm的正方形纸片内剪最大的圆，圆的半径是多少cm．
<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<result>5</result>