Understanding the Concepts

* Newton's Third Law: For every action, there is an equal and opposite reaction. This means that if the spring scale exerts a force on the mass, the mass exerts an equal and opposite force on the scale.

* Force of Friction: When an object moves across a surface, there is a force of friction opposing its motion.

Solving the Problem

1. Identify the Forces:

* Force of the spring scale (F_scale): This is the force the scale applies to the mass, which is 20 Newtons.

* Force of friction (F_friction): This opposes the motion of the mass. We don't know its value yet.

* Force of gravity (F_gravity): This acts on the mass downwards (weight), but it's balanced by the normal force from the table. We don't need to consider it here.

2. Applying Newton's Third Law: Since the scale is pulling the mass with 20 Newtons, the mass exerts an equal and opposite force of 20 Newtons on the scale.

Therefore, the magnitude of the force exerted by the mass on the scale is 20 Newtons.