Given,
Mass of bullet = m1=20g=0.02kg
Mass of pistol = m2=2kg
Initial velocity of bullet = u1=0
Initial velocity of pistol = u2=0
Final velocity of bullet = v1=150ms−1
Let final velocity of pistol = u2=0
By conservation of linear momentum, Initial momentum = Final momentum
m1u1+m2u2=m1v1+m2v2
⇒0=0.02×150+2v2
⇒v2=−150ms−1
Therefore, the pistol recoils with a velocity of 150 m/s towards the left.