A billiard ball moving with a speed of 5 m/s collides with an identical ball, originally at rest. If the first ball stops dead after collision, then the second ball will move forward with a speed of: 

It is a case of elastic collision. As masses of two balls are equal and 2nd ball is at rest before collision, hence after the collision the speeds are just exchanged, i.e., 1st ball will now come to rest while the second ball will move forward with the same speed as that of the first, i.e. 5 $m{s}^{-1}$