When you encounter the word "average", it means that all the items within the consideration has the same number.
When 3 tests has an average score of 80 marks. All the tests are 80, 80 and 80 marks each.
But nte that the three 80 marks each may not be true.
It is assumed to be to make the average correct.
The actual marks may differ from the 80 marks.
They may be 70, 80 and 90 actually, but their average is 80marks.
You may see the post on averaging if you need more information.