You add all the numbers up and then divide the sum by how many numbers you started off with.
So let's say you took 5 tests and these are the marks you got:
95
97
93
84
74
Then, you want to calculate the average:
You add up all the numbers- 443. Then, you divide 443 by 5. That is 88.6. You round it up to 89 and that is your average.
Hope that helped!