Answer
There are 5 normal forces acting on the desk as shown in the figure. The normal force due to the laptop and those due to the floor.
Work Step by Step
According to Newton's third law, a normal force pointing downward is exerted on the desk by the laptop. Since the desk stands on the floor, an upward-pointing normal force is exerted by the floor on the desk. As the contact of the desk on the floor is made through the four legs, this normal force due to the floor will be acting on these legs.
