What is meant by recoiling of a gun?

What is the recoil velocity of a gun having mass equal to 5 kg if a bullet of 25 g acquiress the velocity of 500 m/s after from the gun ?

A shell of mass 0.01 kg fired by a gun of mass 10 kg. If the muzzle speed of the shell is 50 ms^(-1) , what is the recoil speed of the gun?

A shell of mass 0.02 kg is fird by a gun of mass 10 kg. If the muzzle speed of the shell is 600 ms^(-1) , what is the recoil speed of the gun?