The coefficient of restitution $e$ for a perfectly elastic collision is

After a head-on elastic collision between two balls of equal masses,one is observed to have a speed of $3 \, m/s$ along the positive $x$-axis and the other has a speed of $2 \, m/s$ along the negative $x$-axis. The original velocities of the balls are:

$A$ steel ball of radius $2 \,cm$ is at rest on a frictionless surface. Another steel ball of radius $4 \,cm$ moving with velocity of $81 \,cm \,s^{-1}$ collides elastically with the ball which is at rest. After collision, the ball with radius of $2 \,cm$ moves with a speed of:

$A$ body of mass $m$ moving with velocity $v$ collides with a stationary body of mass $2m$. The fraction of kinetic energy lost by the body of mass $m$ is:

$A$ neutron moving with velocity $u$ collides elastically with an atom of mass number $A$. If the collision is head-on and the initial kinetic energy of the neutron is $E$, then the final kinetic energy of the neutron after the collision is:

$Assertion$: $n$ small balls each of mass $m$ collide elastically each second on a surface with velocity $u$. The force experienced by the surface is $2mnu$.
$Reason$: On elastic collision,the ball rebounds with the same velocity.