How many ways are there to put 5 identical balls in 3 different boxes (some boxes can be left alone).
把5个相同的球分放在3个不同的盒子里(有的盒子可以不放)，共有多少种方法．
<gadget id="calculator">1 + 2 + 3 + 4 + 5 + 6</gadget>
<output>21</output>

<result>21</result>