A cylindrical container filled with water has a bottom diameter of 10 cm. An iron ball is immersed in the water. When the iron ball is taken out, the water level drops from 20 cm to 19 cm. Find the volume of this iron ball.
一个装有水的圆柱体容器，底面直径是10厘米，有一个铁球浸没在水中，当取出铁球时，水位由20厘米下降到19厘米．求这个铁球的体积．
<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">20 - 19</gadget>
<output>1</output>

<gadget id="calculator">3.14 * 25 * 1</gadget>
<output>78.5</output>

<result>78.5</result>