Answer : The velocity is given by 4.2 m/s.
Explanation :
It is given that,
Mass of a boy and the skateboard is, m = 65 kg
The momentum of the combined system is, p = 275 kg m/s
We know that the momentum of a body is the product of its mass and the velocity with which it is moving.
i.e.

or



The speed of the boy and the skateboard is 4.2 m/s.