Step 1: Know about the deviation or assumed mean method
The assumed mean method (or deviation) method are just simplified form of
the direct method.
Let x_1, x_2, x_3............, x_n be observations with respective
frequencies f_1, f_2,............f_n
Mean = a + \frac{\Sigma f_id_i}{\Sigma f_i}
Where, d_i = x_i - a
a - Assumed mean
Step 2: Choose one among the observations as the assumed mean, and denote it by 'a'
NOTE: It is taken somewhere in the middle of all the values of observations
Assumed mean a = 57
Step 3: Find the deviation of ‘a’ from each of the observations ( x_i 's)
Deviation d = x_i - a
x_1 = 51, a = 57
d = 51 - 57 = - 6
Calculate the deviation for every observation (See the table)
Step 4; Find the product of d_i(deviations) with the corresponding frequencies f_i, and take the sum of all the f_id_i's.
Calculations are shown in table given below

Step 5: Find the mean of the deviations
Mean of the deviations = a + \frac{\Sigma f_id_i}{\Sigma f_i}
= 57 + \frac{75}{400}
Mean = 57 + 0.1875
Mean = 57.1875