Mass of truck A = 40 kg

Velocity of truck A = 5 m/s

momentum of truck A = 200 kgm/s 

 

Mass of Truck B =50 kg

Mass of boy on it = 10 kg

Velocity of truck B = 10m/s

Momentum of truck B and boy system = 600 kgm/s

 

Just after the first jump

momentum of first truck = 200 kgm/s (same as earlier)

momentum of boy = 10*12= 120 kg m/s (Assuming 2m/s is velocity relative to truck B)

momentum of truck B - 600-120 = 480 kgm/s

Velocity of truck B= 9.6 m/s

 

After boy landing on truck A

Momentum of Truck A boy system = 200+120=320 kgm/s

velocity of truck A and boy system= 320/50=6.4 m/s

Momentum of truck B= 480 kgm/s (same as earlier)

velocity of truck B= 9.6 m/s

 

Just after jump of boy from truck A

momentum of boy = 10*(6.4+2) = 84kgm/s (2m/s is velocity relative to truck A)

Momentum of truck A = 320-84=236 kgm/s

Velocity of truck A = 236/40 = 5.9 m/s

Velocity of truck B= 9.6m/s (same as earlier)

 

After boy lands on truck B

Velocity of truck A = 5.9 m/s (same as earlier)

Momentum of truck B and boy system=480+84=566 kgm/s

Velocity of truck B and boy ystem = 564/60 = 9.4 m/s

 

So I think final velocity of B is 9.4 m/s