Let the assumed mean (A) be 100.
Class interval 
Mid value (x_{i}) 
d_{i} = x_{i}  100 
u_{i} = \(\frac{xi100}{20}\) 
f_{i} 
f_{i}u_{i} 
50  70 
60 
40 
2 
18 
36 
70  90 
80 
20 
1 
12 
12 
90  110 
100 
0 
0 
13 
0 
110  130 
120 
20 
1 
27 
27 
130  150 
140 
40 
2 
8 
16 
150  170 
160 
60 
3 
22 
66 




N = 100 
\(\sum\)f_{i}u_{i }= 61 
A = 100, h = 20
Mean = A + h x \(\frac{\sum f_iu_i}{N}\)
\(=100+20\times\frac{61}{100}\)
\(=100\,+12.2\)
\(=112.2\)