Answer
The force that box A exerts on box B is less than 100 N.
Work Step by Step
The net force on box A is toward the right because it accelerates toward the right. We conclude that the leftward-directed force that box B exerts on box A is less than 100 N, because it cannot cancel the 100-N force.
However, the two forces $\vec{F}_{A\;on\;B}$ and $\vec{F}_{B\;on\;A}$ are always equal in magnitude and opposite in direction. This is a statement of Newton’s third law.
This means that the rightward-directed force that box A exerts on box B is also less than 100 N.
A full solution shows that the force is 25 N toward the right, which is the net force on box B. The net force on box A is therefore 75 N toward the right. The boxes have the same acceleration.
