To determine the force exerted by the 4.0 kg block on the 3.0 kg block, we can apply Newton's third law, which states that for every action, there is an equal and opposite reaction.
Since the 20 N force is applied to the 3.0 kg block, according to Newton's third law, the 3.0 kg block exerts a 20 N force backward on the 4.0 kg block.
Therefore, the force exerted by the 4.0 kg block on the 3.0 kg block is also 20 N, but in the opposite direction.
