We know how by neglecting the air resistance, the problems of projectile motion can be easily solved and analysed. Now we consider the case of the collision of a ball with a wall. In this case the problem of collision can be simplified by considering the case of elastic collision only. when a ball collides with a wass we divide its velocity into two components, one perpendiclar to the wall and other parallel to the wall. If the collision is elastic then the perpendicular component of velocity of the ball gets reversed with the same magnitude. The other parallel component of velocity will remain constant if wall is given smooth. Now, let us take a problem. Three balls A, B, and C are projected from ground with same speed at same angle with the horizontal. The ball A,B and C collide with the wall during their flilght in air and all three collide perpendicularly with the wall as shown in figure. If the total time taken by the ball A to fall back on ground is 4 seconds and that by ball B is 2 secons. Then, the maximum height attained by ball B from ground will be