a.What is the magnitude of N at the lowest point?
b.What is the magnitude of N if the wheel's speed is doubled?
When the student is at the highest point he feels lighter. This makes sense, right? Have you been on a ferris wheel or an elevator? If so, then you know this makes sense. It is good to think about these kinds of things before answering a question.
So instead of feeling the normal force of 656 N, pushing against him, by the seat (by Newton's 3rd law), he feels only 551 N. We know the only force that can be causing this is the centrifugal force due to ...
Solution explains the forces contributing to the normal force.