Cut a circle into several parts along the radius, and assemble them into an approximate rectangle. Given that the length of the rectangle is 40 cm, what is the circumference of the circle?
将一个圆沿半径剪成若干份，再拼成一个近似的长方形．已知这个长方形的长是40厘米，那么，这个圆的周长是多少？
<gadget id="calculator">40 * 2</gadget>
<output>80</output>

<result>80</result>