ball falls into a cylindrical glass jar filled with water. The diameter of the bottom of the glass jar is 20 cm, and four-fifths of the volume of the ball is immersed in the water. If the ball is taken out of the water and the water level in the tank drops by 2 cm, find the volume of the ball.
一个皮球掉进盛有水的圆柱形玻璃缸内，玻璃缸的底面直径是20厘米，皮球五分之四的体积浸入水中．若把皮球从水中取出，缸内水面下降2厘米，求皮球的体积．
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 ** 2</gadget>
<output>100</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">2 / (4/5)</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">3.14 * 100 * (5/2)</gadget>
<output>785</output>

<result>785</result>