While standing in a low tunnel, you raise your arms and push against the ceiling with a force of 100 n. your mass is 70 kg. what force does the ceiling exert on you?

This is a manifestation of Newton's third law of motion called the Law of Action-Interaction. It states that when a body is at rest or not moving, for every force exerted on the object, an equal, opposite force is exerted also towards you. Thus, the ceiling also exerts an equal force of 100 Newtons.