From the 9 numbers 1--9, choose 3 numbers so that their sum can be divisible by 3. How many different ways are there to choose the numbers?
从1--9这9个数中，选3个数使它们的和能被3整除，则不同的选数法有多少种．
<gadget id="calculator">3 * 3 * 3</gadget>
<output>27</output>

<gadget id="calculator">27 + 1 + 1 + 1</gadget>
<output>30</output>

<result>30</result>