Question

A ball of 0.50 KG moving at a speed of 5.0 metre per second collides with another ball of mass 1.0 kg after the collision the ball stick balls stick together and remains motionless the velocity of this 1.0 kg block just before the collision is

Answer 1

Use the law of conservation of momentum to find the velocity of 1kg ball.
Solution is in the attachment.

Hope it helps!!