There are 20 apples, each person divides at least 3 apples, and distributes to 3 people, how many ways are there?
有20个苹果，每人至少分3个，分给3人，有多少种分法．
<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>