A ‘more than’ cumulative frequency distribution of the daily sales (in dozens of notebooks) at a shop is given below:
Lower-class limit (dozens): 0, 10, 20, 30, 40, 50
More than cumulative frequency: 60, 56, 45, 30, 12, 0
What is the actual frequency for the class interval 20–30?
(a) 30
(b) 11
(c) 15
(d) 45