Distribute 20 apples to 3 children, and each person should distribute at least 3 apples. How many different methods can there be?
把20个苹果分给3个小朋友，每人最少分3个，可以有多少种不同的分法？
<gadget id="calculator">13 * 12</gadget>
<output>156</output>

<gadget id="calculator">156 / 2</gadget>
<output>78</output>

<result>78</result>