$15-69.$ The three balls each have a mass $\backslash ($ m $\backslash ).$ If $\backslash ($ A $\backslash )$ has a speed $\backslash ($ v $\backslash )$ just before a direct collision with $\backslash ($ B $\backslash ),$ determine the speed of $\backslash ($ C $\backslash )$ after collision. The coefficient of restitution between each pair of balls is $\backslash ($ e $\backslash ).$ Neglect the size of each ball.

$15$