cuboid with a length and width of 20cm is filled with water. Now put a black iron ball, take out the black iron ball, put a white iron ball in it, and then take out the white iron ball. At this time, the water level drops by 3cm. The second overflow water is half of the first overflow water, what is the volume of the black iron ball in cm**3.
一个长方体，长和宽均为20cm，里面蓄满水，现放一个黑色铁球，拿出黑色铁球后再放一个白色铁球，再取出白色铁球，这时水面下降3cm，已知第二次溢出水是第一次溢出水的一半，则黑色铁球的体积是多少cm**3．
<result>800</result>