We may calculate class marks (x_{i}) for each interval by using the relation
x_{i} \(=\frac{Upper\,class\,limit+lower\,class\,limit}{2}\)
Now taking assumed mean (A) = 16
Number of days 
Number of students (f_{i}) 
x_{i} 
d_{i} = x_{i}  16 
f_{i}d_{i} 
0  6 
11 
3 
13 
143 
6  10 
10 
5 
8 
80 
10  14 
7 
12 
4 
28 
14  20 
4 
16 
0 
0 
20  28 
4 
24 
8 
32 
28  38 
3 
33 
17 
51 
38  40 
1 
39 
23 
23 

N = 40 


\(\sum\)f_{i}d_{i} = 145 
Mean (x̅) = A + \(\frac{\sum fidi}{N}\)
\(=16+\frac{145}{40}\)
\(=16  3.62=12.38\)
So,
mean number of days is 12.38 days for which students were absent.