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We know that there is a one-to-one correspondence between the $r$ with $0\le r\lt20$ with $gcd(r,20)=1$ and the pairs $(r_1,r_2)$ where $0\le r_1\lt4$, where $0\le r_2\lt5$ such that $gcd(r_1,4)=1$ and such that $gcd(r_2,5)=1$. Exhibit this correspondence.

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    A lot like your other question.2012-10-12

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