Answer:
Consider a class in which there are 40 students .There is a Mathematics test of 50 marks .Two students were absent.The Duration of test was for one and half an hour.
So Marks of students according to their roll number are 12,8,4,20,25,36,2,29,35,39,5,48,43,37,38,29,17,15,15,25,31,32,26,28,27,24,2,15,34,,50,46,48,37,28,26,45,49,24.
So in this case it would be better to prepare frequency polygon by selecting a class interval.
Class interval = 0-5,5-10,10-15,15-20,20-25,25-30,30-35,35-40,40-45,45-50.
Or 0-10,10-20,20-30,30-40,40-50.
or you can make class interval by yourself.