jmc

Both $m$ and $n$ can be written in the form $11k+5$. For $m$, we have that $11k+5\ge 100$, so $k\ge \frac{95}{11}$, so since $k$ must be an integer, we have that $k=9$, so $m=11(9)+5=104$. For $n$, we have that $11l+5\ge 1000$, so $l\ge \frac{995}{11}$, so since $l$ must be an integer, we have that $l=91$, so $n=11(91)+5=1006$. Therefore, $n-m=1006-104=\boxed{902}$.