The table shows a grouped frequency distribution of the number of books read by 25 students:
Class Interval (books) | Frequency
1–5 | 3
6–10 | q
11–15 | 8
16–20 | 6
Which of the following equations must q satisfy so that the table represents exactly 25 observations?
(a) 3+q=25
(b) q+8+6=25
(c) 3+q+8+6=25
(d) 3+8+6=q