A ball $A$ of mass $2m$ is at the top of a frictionless shute at height $5h$ above the ground and another ball $B$ of mass $m$ is at rest on the horizontal surface at the end of the shute at height $h$ above the ground. Ball $A$ starts to slide from rest down the shute and hits ball $B$ which is projected, after the impact, off the end of the shute. If the coefficient of restitution between the balls is $0.5$, how far does ball B go horizontally before it hits the ground?