The class mark of the class interval 10-25 is:

1. A class interval is treated as if it were centred on its mid-point. 2. The mid-point of a class, also called its class mark, is the average of its upper and lower limits. 3. Here that is (10 + 25) / 2. 4. So the class mark is 17.5. 5. 15 is the width of the class, not its centre, which is the usual slip. _Source: syllabus Maths, Ch 13 'Statistics', S13.2 Mean of Grouped Data_