A histogram shows the number of pages read by 50 students in a reading challenge. The class intervals (in pages) are 0–10, 10–20, 20–30 and 30–40. The heights of the bars are 2 cm, 4 cm, 3 cm and 1 cm respectively. The vertical axis is scaled so that 1 cm corresponds to 5 students. Calculate the average (mean) number of pages read by the students.