Explanation:

The rifle and the bullet make a system. Since, there is no external force acting on it the momentum of the system should be conserved.

∴m

1

u

1

+m

2

u

2

=m

1

v

1

+m

2

v

2

⇒4×0+(

100

50

)×0=4v+

1000

50

×35

⇒4v=−

20

35

⇒v=−

80

35

m/s

Note:- Negative sign indicates that recoil velocity is opposite to the motion of the bullet.