In an elastic collision of two particles,which of the following quantities is conserved?

$A$ heavy steel ball of mass greater than $1\, kg$ moving with a speed of $2\, m/s$ collides head-on with a stationary ping-pong ball of mass less than $0.1\, g$. The collision is elastic. After the collision,the ping-pong ball moves approximately with a speed of ......... $m/s$.

There is a head-on elastic collision between two particles moving in the same direction with speeds $5 \,m/s$ and $3 \,m/s$ respectively. After the collision,the velocity of the first particle becomes $4 \,m/s$ in the same direction. What is the velocity of the second particle?

$A$ 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:

$A$ ball is dropped from height $h$ on a plane. If the coefficient of restitution of the plane is $e$ and the ball hits the ground two times,the height up to which it reaches after two jumps will be:

$A$ body of mass $5 \ kg$ makes an elastic collision with another body at rest and continues to move in the original direction after collision with a velocity equal to $\frac{1}{10}$ th of its original velocity. Then the mass of the second body is (in $kg$)