In the elastic collision of objects

Which of the following statement is true?

Two bodies having same mass $40\, kg$ are moving in opposite directions, one with a velocity of $10$$m/s$ and the other with $7\,m/s.$ If they collide and move as one body, the velocity of the combination is ........ $m/s$

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

A body of mass $M$ moves with velocity $v$ and collides elastically with a another body of mass $m$ ($M>>m$) at rest then the velocity of body of mass $m$ is

A mass of $20\, kg$ moving with a speed of $10\,m/s$ collides with another stationary mass of $5\,kg.$ As a result of the collision, the two masses stick together. The kinetic energy of the composite mass will be ............. $J$