Question

A ball of mass 'm' moving with speed v makes head on collision with an identical ball at rest.The kinetic energy of ball after collision is three fourth of original . The coefficient of restitution is?

Answer 1

$e = \frac{v2 - v1}{u2 - u1}$
Use this formula with Kinetic energy theorem and get the answer.
If you are still confused, then just comment below.