Cut out a fan shape from the paper with a radius of 30 cm and a central angle of 120 degrees. Use this fan shape to make the side of a cone, and add a bottom surface. What is the diameter of the bottom surface in centimeters?
从纸上剪下一个半径是30厘米、圆心角是120度的扇形，用这个扇形做一个圆锥的侧面，另外再配一个底面，这个底面的直径是多少厘米．
<result>20</result>