Calculating the average of any set of numbers is always found by ADDING all the numbers TOGETHER and dividing that number by however many numbers you had.
Eg To calculate the average of 5 test scores take the five scores 80%, 95%, 100%, 77%, 90%, and add of the 5 scores. The total of the five scores total is 442.
Then calculate the average of the 5 scores by taking the total of 442 and dividing that number by five (you have added up 5 different numbers) giving you a sum of 88.4%