Explanation:
The recoil velocity is the velocity by which pistol bounces back when a bullet is fired.
In this case, the momentum of the bullet is equal to the momentum of the pistol.
Hence, mbvb = mpvp (where mb and mp are the masses of bullet and pistol respectively and vb and vp are the velocities of bullet and pistol respectively).
3 x vp = 40 x 10-3 x 150
vp = 40 x 10-3 x 50
vp = 2000 x 10-3
vp = 2 m/s
Here, the velocity of pistol is taken with negative sign because, it is in the opposite direction to that of the velocity of the bullet, i.e., bullet moves forward and pistol moves backwards.