identical balls are placed in three different boxes, at least one in each box, how many different ways are there?
7个相同的球，放入三个不同的盒子里，每个盒子至少放一个，不同的放法有几种？
<gadget id="calculator">6 * 5</gadget>
<output>30</output>

<gadget id="calculator">2 * 1</gadget>
<output>2</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<result>15</result>