Two boys on skates push off from each other. The 40 kg boy moves to the left at 10 m/s. If the other boy moves to the right at 8 m/s, what is his mass?

Answer 1

Momentum of the 'two boys' system shall be conserved (because there is no external force in the horizontal direction)

So, m1v1 = m2v2
m1 = 40*10/8 = 50 Kg

Answer 2

Thank you