Step 1: Recall the formula of the mean of the grouped data

NOTE: The mean (or average) of observations is the sum of the values of

all the observations divided by the total number of observations.

Let x_1, x_2, x_3............, x_n be observations with respective

frequencies f_1, f_2,............f_n

Mean = \frac{f_1x_1 + f_2x_2 + ........f_nx_n}{f_1 + f_2 +..........+ f_n}

or

Mean = \frac{\Sigma f_ix_i}{\Sigma f_i}

Step 2: Re-organize given data in the table and find the sum of all observations.

Sum of the values of all the observations \Sigma f_ix_i = 499

Total number of observations \Sigma f_i = 40

So, the mean = \frac{499}{40}

Mean = 12.4