A boy on a 2.2 kg skateboard initially at rest

tosses a(n) 7.7 kg jug of water in the forward
direction.
If the jug has a speed of 2.9 m/s relative to
the ground and the boy and skateboard move
in the opposite direction at 0.57 m/s, find the
boy’s mass.
Answer in units of kg

(I just want the formula) due in about 2 hours

2 answers

Not sure, but I believe it is (M+M)V=MV. Solve for your unknown.
(M+2.2Kg)(0.57m/s)=(7.7Kg)(2.9m/s)