Final answer:
The magnitude of the action-reaction pair between two boxes cannot be determined from their masses alone. Newton's third law indicates that the forces are equal and opposite, but without additional details, we cannot calculate a specific value for the forces.
Step-by-step explanation:
The situation described involves two boxes, A and B, with masses of 7 kg and 3 kg respectively. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. This means the magnitude of the action-reaction pair between the two boxes is equal and opposite.
However, without additional information, such as forces applied or accelerations, we cannot calculate the specific magnitude of the action-reaction pair force.
In physics, the only scenario where we could directly calculate the force without more details would be if this were a gravity-related question, where the formula for gravitational force (F = G*(m1*m2)/r^2) would apply, but this is not the case here.
Moreover, the reaction pair does not have a combined magnitude that is simply the sum of the two masses; rather, it refers to the fact that the forces exerted by the two boxes on each other are equal in magnitude.