The mathematics teacher took a measuring cylinder in class, which contained 400 ml of water, and then put 6 glass balls with equal radii, the water level rose to 520 ml, what is the volume of each glass ball?
数学老师上课时拿了一个量筒，里面装有400毫升的水，然后放入6颗半径相等的玻璃球后，水面上升到520毫升，每颗玻璃球的体积是多少？
<gadget id="calculator">520 - 400</gadget>
<output>120</output>

<gadget id="calculator">120 / 6</gadget>
<output>20</output>

<result>20</result>