Although most people use the term average for this type of calcuation:
Four tests results: 15, 18, 22, 20
The sum is: 75
Divide 75 by 4: 18.75
The 'Mean' (Average) is 18.75
(Often rounded to 19)
The truth of the matter is that the above calculation is considered the arithmetic mean, or often referred to as the mean average.