We may calculate class marks (x_{i}) for each interval by using the relation
x_{i} \(=\frac{Upper\,class\,limit+lower\,class\,limit}{2}\)
Class size = 0.04
Now taking 0.14 as assumed mean (A), we can calculate as follows:
Concentration So_{2} (ppm) 
Frequencey (f_{i}) 
x_{i} 
d_{i} = x_{i}  0.14 
ui 
f_{i}u_{i} 
0.00  0.04 
4 
0.02 
0.12 
3 
12 
0.04  0.08 
9 
0.06 
0.08 
2 
18 
0.08  0.12 
9 
0.10 
0.04 
1 
9 
0.12  0.16 
2 
0.14 
0 
0 
0 
0.16  0.20 
4 
0.18 
0.04 
1 
4 
0.20  0.24 
2 
0.22 
0.08. 
2 
4 

N = 30 



\(\sum\)f_{i}u_{i} = 31 
Mean (x̅) = A + \(\frac{\sum f_iu_i}{N}\times h\)
\(=0.14+\frac{31}{30}\times(0.04)\)
\(=0.140.04133\)
\(=0.099\)ppm
SO,
mean concentration of SO_{2} in the air is 0.099 ppm