RMO 2013 M4

In a triangle $ABC$ , points $D$ and $E$ are on segments $BC$ and $AC$ such that $BD=3DC$ and $AE=4EC$ . Point $P$ is on line $ED$ such that $D$ is the midpoint of segment $EP$ . Lines $AP$ and $BC$ intersect at point $S$ . Find the ratio $BS/SD$ .