# Exhibit this correspondence.

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.

-
A lot like your other question. –  mick Oct 12 '12 at 14:01