Given, Mass (m_{1}) of bullet = 20 g = 0.02 kg

Mass (m_{2}) of pistol = 2 kg

Initially bullet is inside the gun and it is not moving.

Mass = m_{1} + m_{2} = (0.02 + 2) kg = 2.02 kg

And u_{1} = 0

So, Initial momentum = 2.02 × 0 = 0 .....(i)

Finally let the velocity of pistol be v_{2} and v_{1} for bullet = 150

Final momentum = m_{1}v_{1} + m_{2}v_{2}

= 0.02 × 150 + 2v_{2} ...(ii)

We know that

Initial momentum = Final momentum