The electric field intensity generated by a single point charge is given by
where
k is the Coulomb's constant
q is the charge
r is the distance from the charge
In this problem, the charge is 
and we are asked to calculate the field at distance 
, so the electric field is
The answer is 12.5 kg because 250N / 20m/s^2
I hope that helped
according to newton's third law, every action has equal and opposite reaction. in this scenario of making a jump from the boat onto a dock, as i jump my feet in contact with the boat push the boat in backward direction. hence the action force is the push by my feet on the boat. the boat reacts by applying a reaction force on my feet pushing the feet in forward direction. hence reaction force here is the force by the boat on the feet. due to the reaction force of the boat on feet, i am pushed in forward direction to reach the the dock.
