Explanation:

There are 2 ways to solve this problem.
The difference between every term is 8, hence we can say that they are in Arithmetic Progression. First-term = 8, common difference = 8, hence we can find the 20th term using the formula
T20 =a+(n-1)d = 8 + (20 - 1)8
= 8 + 19 * 8
= 160
We can find the Average of Arithmetic Progression using the formula, Average = (First Term + Last term) / 2
= (8 + 160) / 2

= 168 / 2
= 84
The other way is to directly find the sum of first 20 terms using the formula Sn = n/2 [2a + (n-1)d]
S20 =(20/2)[2*8+(20-1)*8]
Therefore the sum of 20 terms = 1680
The average of 20 terms = sum of 20 terms / number of terms = 1680 / 20
= 84