Class marks of a distribution are 26, 31, 36, 41, 46, 51, 61, 66, 71. The lower and upper class limits of the first class interval are
23.5 - 28.5
Class marks are uniformly spaced, so the class size is the difference between any two consecutive class marks.
Class size = 31 - 26 = 5
If a is the class interval of size h, then the lower and upper limits of the class interval are a – h/2 and a +h/2
Lower limit of first class interval = 26 – 5/2 = 23.5
And upper limit of first class interval = 26 + 5/2 = 28.5