12. Show that if $r=\left(a_{k} a_{k-1} \ldots a_{1} a_{0}\right)_{b}$, then the quotient and remainder when $n$ is divided by $b^{j}$ are $q=\left(a_{k} a_{k-1} \ldots a_{j}\right)_{b}$ and $r=\left(a_{j-1} \ldots a_{1} a_{0}\right)_{b}$, respectively.