Answer:
vf = 34.5 [m/s]
Explanation:
This problem considers the study of the conservation of the linear moment, momentum can be calculated using its definition.
p = m * v
where:
p = momentum [kg*m/s]
m = mass [kg]
v = velocity [m/s]
Since the moment is conserved, i.e. the momentum at the beginning of the action will be equal to the moment at the end of the movement
The initial mass is the sum of the masses of all children and the speed will be given in the initial conditions.
When the kid A jumps at a speed of zero, only we have the masses of children B & C. In this way we can determine their velocities at the end.
(42.8+31.5+25.9) * (19.8) = (42.8*0) + (31.5+25.9)* vf
vf = 34.5 [m/s]