If a ball is given an initial speed of 15 m/s on a horizontal, frictionless surface, how fast will the ball be rolling in 5 seconds?

The surface is frictionless, so there is no frictional force acting on the ball. There are no other forces acting on the ball in the horizontal direction, so it's a uniform motion with constant speed. Therefore, the velocity of the ball will remain the same for the entire duration of the motion, and so after 5 seconds the velocity is still 15 m/s.