Given information:
The arithmetic mean of the given data is 20.6.

To find:
The value of 'a'.

Approach:
To find the value of 'a', we need to know the complete data set. However, the data set is not provided in the question. Therefore, we cannot determine the value of 'a' based on the given information alone.

Explanation:
The arithmetic mean, also known as the average, is calculated by summing up all the values in a data set and dividing it by the total number of values.

Formula:
Arithmetic Mean = Sum of all values / Total number of values

Since the complete data set is not given, we cannot calculate the sum of all values or the total number of values. Therefore, it is not possible to determine the value of 'a' based solely on the arithmetic mean.

Conclusion:
Without the complete data set, it is not possible to determine the value of 'a' based on the given information. It is important to have the complete data in order to calculate the arithmetic mean accurately.