Answer: It Is 60 kg*m/s.
Explanation: The change in momentum is equal to the mass of the cart multiplied by the change in its velocity.
The initial momentum of the cart is zero, since it is at rest. The final momentum of the cart is equal to its mass, 20 kg, multiplied by its final velocity, 3 m/s. So, the change in momentum is:
Change in momentum = (20 kg)*(3 m/s) = 60 kg*m/s
So the answer is 60 kg*m/s