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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?