The resultant force on the animal = Resultant mass total acceleration
F = 0.2 2.5 to the right
F = 0.5 to the right.
As, girl exerting a force of 3.5 N & it's not mentioned that she is in right or left, so the force exerting by boy would be either:
3.5-0.5 = 3 OR 3.5+0.5 = 4
If boy exerting a greater force then, answer will be 4 N & if girl exerting a greater force the, answer will be 3 N
Hope this helps!
