[計量] magoosh: 判別3的倍數
原題不難，但主要是他用了一個解題技巧，如下：
要判別一個數是否為3的倍數(ex:759325)
我們可以把各個數字加總:7+5+9+3+2+5=31
31不能被3整除→759325不能被3整除。
請教這個方法的原理是什麼啊！先感謝回答~
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