Given,
Mean = 18
Let missing frequency be V
Class interval 
Mid value (x_{i}) 
f_{i} 
f_{i}x_{i} 
11  13 
12 
7 
84 
13  15 
14 
6 
84 
15  17 
16 
9 
144 
19  21 
20 
V 
20V 
21  23 
22 
5 
110 
23  25 
24 
4 
56 


N = 44 + V 
\(\sum\)f_{i}x_{i} = 752 + 20V 
Mean = \(\frac{\sum f_ix_i}{N}\)
\(18 = \frac{752+20V}{44+V}\)
792 + 18V = 752 + 20V
792 – 752 = 20V – 18V
40 = 2V
V = 20