In an inelastic collision, what is conserved

A ball loses $15.0\%$ of its kinetic energy when it bounces back from a concrete wall. With what speed you must throw it vertically down from a height of $12.4\, m$ to have it bounce back to the same height (ignore air resistance)? ............. $\mathrm{m} / \mathrm{s}$

When two particles collide elastically, then impulsive force can't change

A body of mass $2\,kg$ makes an elastic collision with a second body at rest and continues to move in the original direction but with one fourth of its original speed. What is the mass of the second body? ................ $ \mathrm{kg}$

Two small balls are fixed at the ends of a light rigid rod of length $0.4\  m$.The system is released from height $h = 5\  m$ with rod horizontal. The balls collide with the horizontal surface and rebound . The coefficient of restitution between $A$ and ground is $0. 6$ and that between $B$ and ground is $0.4$ . Find angular speed (in $rad/s$) just after the collision. Taking acceleration of free fall $10\ m/s^2$

In an elastic collision of two particles the following is conserved