The class marks of a frequency distribution are 15, 20, 25, 30…..
The class corresponding to the class mark 20 is
Class marks = 20 (given)
Class size = 20 – 15 = 5
Lower class limit = (20 – 5/2 ) = 20 – 2.5 = 17.5
Upper class limit = (20 + 5/2) = 20 + 2.5 = 22.5.
Hence,
Class mark 20 will lie in class 17.5 – 22.5