A50 kg box is at rest on a horizontal surface. when an 80 n force is applied to the box, it slides on the surface with a constant velocity. how will the force of friction change if an identical box is placed on top of the 50 kg box

As the box is moving with a constant velocity, the two forces acting on the box are canceling each other.

Then   friction force = 80 Newtons              but in the opposite direction.

Friction force =  Mu  * Normal force exerted by ground  =  Mu * weight of box
So we find Mu.
Mu = coefficient of friction between box and horizontal surface
           = Force of friction / weight  =  80 / 50 * 9.81 = 0.163

When an identical box is placed on top, the force of friction is
       = Mu * total weight = 0.163 * (50+50) * 9.81 = 159.9 Newtons