Mean is the sum of the values divided by how many there are. 4, 6, and 8 average to 18/3 = 6. Mean, median, and mode as a set are in mean, median, and mode.
Add, then divide
A missing value that makes the mean 10 is an algebra question: sum = mean × count. The nth arithmetic term is a different leftover, in nth term of an arithmetic sequence.
The short list is in ACT Math formulas.
The bottom line
Mean = sum / count. Median is the middle, not this formula.
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