The teacher wants to distribute 20 identical apples to 3 children, requiring each child to get at least 3 apples, so how many distribution methods are there?
老师要将20个相同的苹果分给3个小朋友，要求每个小朋友至少分得3个苹果，那么共有多少种分配的方法？
<gadget id="calculator">12 + 1</gadget>
<output>13</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">13 * 6</gadget>
<output>78</output>

<result>78</result>