In a game of pool, a 0.4 kg cue ball is traveling at +0.80 m/s when it hits a slower striped ball moving at +0.38 m/s. after the collision, the striped ball moves off at +0.62 m/s.

what is the magnitude of the final velocity of the cue ball? assume all pool balls have the same mass.

This can be answered through the concept of conservation of momentum and assuming that the cue ball and the striped ball have the same masses. Such that,
                        m1v1 + M1V1 = m2v2 + M2V2
Substituting the known values,
        (0.4 kg)(0.80 m/s) + (0.40 kg)(0.38 m/s) = (0.40 kg)(0.62 m/s) + (0.40 kg)(x m/s)
The value of x from the equation above is,
                                 x = 0.56 m/s