Condition for a Line to Lie in a Plane

Q. $ABCD$ is a tetrahedron and $O$ is any point. If the line joining $O$ to the vertices meets the opposite faces at $P,Q,R$ and $S$, then the value of
$\frac{OP}{AP}+\frac{OQ}{BQ}+\frac{OR}{CR}+\frac{OS}{DS}$ is