有甲、乙、丙三个互相咬合的齿轮，当甲齿轮转了2圈时，乙齿轮正好转了3圈，丙齿轮正好转了4圏，这三个齿轮齿数之比是多少？

分析：齿数越多，转的圈数越少；齿数越少，转的圈数越多。转的圈数与齿数成反比，所以，甲、乙、丙三个齿轮齿数之比为4：3：2，这恐怕是最容易想到的答案了，但很遗憾，这是错误的答案。我们可以这样检验：如果答案真是4：3：2的话，我们可以将齿数的份数与圈数相乘，结果得到8份、9份、8份，而实际上这三个齿轮的齿数份数与圈数的积应该是相等的。

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