Cuanta fuerza se necesita lara evitar que una piedra de 20N caiga?

Reaction is 20N.

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.