3. Eventually you will have negative numbers when working with a decreasing pattern. There may be no negative absences. The only way is if those are leading into the people who are present.
4. 15 segments. n=n x (n-1)/ 2
5. Use the formula. 10 x (10-1) / 2
10 x 9 / 2
90/ 2
45 segments in a figure with 10 points.