Let $ABCD$ a generic convex quadrilateral. Let $P,Q,R,S$ the midpoints of $AB,BC,CD$ and $AD$ respectively. The segments $PR$ and $QS$ intersect each other in a point $O$, and divide $ABCD$ in 4 sub-quadrilaterals. If the area of $APOS$ is 25, the area of $PBQO$ is 16 and the area of $QCRO$ is 36, then what is the area of the remaining quadrilateral $SORD$ ?
I know that $PQRS$ is a parallelogram, so that $O$ is the midpoint of the segments $PR$ and $QS$, and its area is half the area of $ABCD$, but I can't move on from this. Any help?