If 6 identical balls are placed in four different boxes A, B, C, and D, if each box is not empty, how many different ways are there in total?
6个相同的球，放在A、B、C、D四个不同的盒内若每个盒内都不空，共有多少种不同的放法．
<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">4 * (3/2)</gadget>
<output>6</output>

<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<result>10</result>