aboy having a mass of 75 kg holds in his hands a bag of flour weighing 40N with what force does the floor Push up on his feet ?​

40 newtons

Explanation:

It will be equal to the amount of force pushing down because otherwise he'd just fall through the floor.

Heres newtons law for the principle

Newton's third law is: For every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object.