The radius of a wheel is 25 cm. When the wheel rolls on the ground in a straight line, how much does the axis of the wheel move?
一个轮子的半径是25厘米，当轮子在地面上沿直线滚动一周时，轮子的轴心大约移动了多少．
<gadget id="calculator">2 * 3.14 * 25</gadget>
<output>157</output>

<result>157</result>