An interesting quadrilateral

The $ABCD$ trapezoid is right-angled. Sides $AB$ and $CD$ are parallel. $AB=9$ and $AD=10$ . The midpoints of diagonals $AC$ and $BD$ are $P$ and $Q$ respectively. The length of $PQ$ is 4. The bisector of the $\angle BAD$ intersects $BC$ at point $M$ . If $CM/MB=1/n$ , what is $n$ ?