When the cardboard is at rest, the magnitudes of the pair forces acting on it are equal, and their directions are opposite. This is a direct consequence of Newton's Third Law of Motion.

Here's a breakdown:

* Newton's Third Law: For every action, there is an equal and opposite reaction.

* Pair Forces: These are the forces that two objects exert on each other.

* Cardboard at Rest: Since the cardboard is not moving, the net force acting on it must be zero.

Let's consider an example:

Imagine the cardboard is sitting on a table.

* Action: The cardboard exerts a downward force on the table due to its weight (gravity).

* Reaction: The table exerts an upward force on the cardboard, called the normal force, which is equal in magnitude to the cardboard's weight but opposite in direction.

These two forces are the pair forces acting on the cardboard. They are equal in magnitude and opposite in direction, ensuring that the net force on the cardboard is zero, keeping it at rest.

Important Note: This principle applies to all pairs of forces acting on any object at rest. It's a fundamental concept in understanding how forces interact in the world around us.