Let us find the mean of the following data by step deviation method.

To find mean using step-deviation method, we need to create a table showing frequencies, midpoints and deviations.

Observe that class size, h = (30 – 0) = (60 – 30) = … = (150 – 120) = 30

We have,

A = 75

∑f_iu_i = 2

∑f_i = 80

h = 30

By step deviation method, we can give the formula of mean as:

Using the above values, we have

⇒

⇒

⇒

⇒

⇒ Mean = 75.75

Thus, mean is 75.75.