A measuring cylinder holds 280 ml of water. After putting in 1 glass marble, the water level rises to the place where the scale is 300ml. What is the volume of the glass marble.
一个量筒，盛有280毫升的水．放入1颗玻璃弹珠后，水面上升到刻度是300毫升的地方．这颗玻璃弹珠的体积是多少．
<gadget id="calculator">300 - 280</gadget>
<output>20</output>

<result>20</result>