Thank you for the opportunity to help you with your question!
Horizontal component of throw V = (cos 40) x 20, = 15.32m/sec.
Vertical component has no affect on the problem, because of lack of friction.
His momentum will be the momentum given the mass, which is 15.32kg/m/sec. (1kg x 15.32m/sec.).
Momentum is conserved.
Please let me know if you need any clarification. I'm always happy to answer your questions.