Problem

Two uniform smooth spheres $A$ and $B$ of equal radii have masses $4m$ and $m$ respectively. Sphere $B$ is at rest on a smooth horizontal surface. Sphere $A$ is moving on the surface with speed $u$ and collides directly with sphere $B$. After the collision, the momentum of $A$ is three times the momentum of $B$.

Find the value of the coefficient of restitution $e$.