Answer:
Reaction is 20N.
Step-by-step explanation:
In general, Newton's third law of motion explains that when a force is applied on an object, a reaction of equal magnitude applies in the reverse direction of the force.
i.e F = -R
This implies that, reaction is a force in the opposite direction to that of the force applied.
Force can also be related to the weight of an object as;
F = W = mg
Thus to prevent the 20N stone from falling, a force of 20N is applied in the opposite direction. So the reaction, R is 20N.