A boy sitting on a chair placed on floor of a room involves two action-reaction pairs of forces, one between the boy and chair, second with the whole system.

(a) Firstly, the force between the boy and the chair. The action force (F = mg) is the force exerted by the boy on the chair towards downward direction and the reaction force (F' = −mg) chair applies in upward direction as shown in fig(a)

(b) Section, the force between the boy, chair and floor The action force (F = (M + m) g) exerted by the boy and the chair towards downwards on floor and the reaction force (F' = (M + m) g exerted by the floor on the chair and boy towards upwards direction as shown in fig(b)