Answer:
Find the outliers, then exclude them in the calculation.
Explanation:
Calculating the mean is to take the average of a series of values.
Outliers would be the values that don't necessarily fit, sort of like outcasts in the series.
For example,
If you had 10 tests in a year, and 8 of them you got between 75-90%, and the remaining 2 were below 50%.
In this example it is easy to see that out of the series of values, the 2 test scores below 50% are the outliers.
To calculate the mean without the outliers you would simply not include them in your calculation of the mean. (or average)
So, add up all those 8 test scores between 75-90% and then divide by 8. This would give you your mean without outliers for this set of values.