how to find the mean if the class intervals are unequal??

like fr e- 0-10,10-30,30-35....

When the class intervals are continuous but each of the given classes of unequal width, then we can find the mean with the help of any of the 3 methods : 

Direct method
Assumed mean method
Step deviation method

Consider the following example.

A class teacher has the following absentee record of 40 students of a class for the whole term. Find the mean number of days a student was absent.
 

No. of days	0-6	6-10	10-14	14-20	20-28	28-38	38-40
No. of students	11	10	7	4	4	3	1

Here is the solution.

 

CLASS INTERVAL	FREQUENCY (fi)	CLASS MARKS(xi)	fixi
0-6	11	3	33
6-10	10	8	80
10-14	7	12	84
14-20	4	17	68
20-28	4	24	96
28-38	3	33	99
38-40	1	39	39
Total	∑ fi = 40		∑ fi xi = 499

Mean = ∑ f_i x_i / ∑ f_i = 499/40 = 12.475 $\approx$ 12.48