Suppose $r$ and $s$ are positive integer numbers with $gcd(r,s)=1$, then show that the mapping $\varphi : Z_{rs}\rightarrow Z_r \times Z_s$ with $\varphi (n) = n (1,1)$ is isomorphism ring!
I'll let you show that it's a ring homomorphism. To see that it's an isomorphism, note that $\ker \varphi = \{n \in \mathbb{Z}_{rs} : n(1,1) = (1,1)\} = \{n \in \mathbb{Z}_{rs} : n \equiv 1 \mod r \mbox{ and } n \equiv 1 \mod s\}$ $= \{n \in \mathbb{Z}_{rs} : n \equiv 1 \mod r\} \cap \{ n \equiv 1 \mod s\} = \{1\}$. (Chinese remainder theorem!) So the map is injective. Since we have $|\mathbb{Z}_{rs}| = |\mathbb{Z}_{r} \times \mathbb{Z}_{s}| = rs$ we are done.