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There are three ropes. One of them is $10$ inches long, the other two are $28$ and $30$ inches respectively. If we cut those ropes into small pieces with nothing left and each piece has the same length, how long is each piece at most? 
Overseas Competition->Knowledge Point->Number Theory Modules->Factors and Multiples->Common Factors and Common Multiples->Common Factors and the Greatest Common Factors->The Greatest Common Factor of Multiple Numbers
The greatest common factor of $10$, $28$ and $30$ is $2$. 