Why law of conservation of linear momentum is universal and fundamental law ?

The momentum of a system is conserved

A bullet of mass $50$ gram is fired from a $5 \,kg$ gun with a velocity of $1km/s$. the speed of recoil of the gun is .......... $m/s$

Two identical billiard balls strike a rigid wall with the same speed but at different angles, and get reflected without any change in speed, as shown in Figure. What is

$(i)$ the direction of the force on the wall due to each ball?

$(ii)$ the ratio of the magnitudes of impulses imparted to the balls by the wall ?

A bullet mass $10\, gm$ is fired from a gun of mass $1\,kg$. If the recoil velocity is $5\, m/s$, the velocity of the muzzle is ........ $m/s$

A bomb is projected with $200\,m/s$ at an angle $60^o$ with horizontal. At the highest point, it explodes into three particles of equal masses. One goes vertically upward with velocity $100\,m/sec,$ second particle goes vertically downward with the same velocity as the first. Then what is the velocity of the third one