m = 0.020 kg is the mass of the shell.

M is the gun’s mass, which is 100 kilogrames.

The shell’s speed is 80 m/s.

The shell’s and gun’s initial velocity is both zero. As a result, the system’s initial momentum is zero.

The beginning momentum is equivalent to the end momentum when the rule of conservation of momentum is applied.

So,

0 = MV

v = mv/M is the gun’s recoil speed.

(0.020 x 80)/100 = v

v = 0.016 m/s