Given,
The momentum of object A before the collision, p₁=10 Ns
The momentum of object B before the collision, p₂=3 Ns
The momentum of object A after the collision, p₃=5 Ns
From the law of conservation of momentum, the total momentum of a system always remains constant. Thus the total momentum of the objects before the collision must be equal to the total momentum of the objects after the collision.
Thus,

Where p₄ is the momentum of object B after the collision.
On substituting the known values,

Thus the momentum of the object B after the collision is 8 Ns