Homework: geometric/ numerical puzzle

Thirty equally spaced points on the circumference of a circle are labelled in order with the numbers 1 to 30. Which number is diametrically opposite to 7?

Think of it analogical to a clock with numberd from 1 to 12. But in this example, we are looking at number 1-30. If 15 is opposite to 30, we can work our way down to find 7 is opposite to 22. 

Extensions to this puzzle:

Assume there are 28 points, which number is diametrically opposite to 8? 

Asumme there are 25 points, which number is diametrically opposite to 8?

Which question is possible/impossible to solve? Why is that?

What makes a puzzle truly geometric, rather than simply logical?

The wording of a problem can make a puzzle geometric. In our question, the words "equally spaced" and "diametrically" make it geometric. Although it can be solved with logic, but we can see that it is a question relating to geometry.