Since the maximum number of consumers (is 20) have got monthly consumption in the interval 120-140,
the model class is 120 -140.
The lower boundary (l) of the modal class = 120.
The class size (h) = 20
The frequency of modal class (f1) = 20
The frequency of the class preceding the modal class (f0) = 16.
The frequency of the class succeeding the modal class (f2) = 14.
Now, using the formula: