Let $m$ and $m'$ be positive integers, and $d=gcm(m,m')$.
(i) The system:
$x \equiv b (mod \ m)$
$x \equiv b' (mod \ m')$
has a solution if and only if $b \equiv b' (mod \ d)$
(ii) two solutions of the system are congruent $mod \ l$, where $l = lcm(m,m')$.
I can prove part (i), but can...