Average = sum of the numbers ÷ how many numbers there are
For 4, 8 and 15: add them (4 + 8 + 15 = 27), count them (3), divide (27 ÷ 3 = 9). The average is 9. This is the arithmetic mean, and it is what people mean by "average" unless they say otherwise.
How to find the average of numbers, step by step
1) Write the numbers down. 2) Add them all together to get the sum. 3) Count how many numbers there are. 4) Divide the sum by the count. Zeros count as numbers; blanks do not.
How can I calculate the average of test scores?
Scores of 82, 91, 76 and 88: sum 337, count 4, average 84.25. If some tests are worth more than others, use a weighted average instead.
How to find the average of percentages
If each percentage came from the same-sized group, average them normally. If the groups differ in size, weight each percentage by its group size, otherwise a 50% from 10 people counts the same as a 95% from 900.
How to find the average in Excel or Google Sheets
Type =AVERAGE(A1:A10). It ignores empty cells and text. For a weighted average use =SUMPRODUCT(values, weights)/SUM(weights).
Common mistakes
Dividing by the wrong count (forgetting a number, or counting a blank). Averaging averages without weighting them. Reporting the mean when one huge outlier distorts it; in that case the median is the better "typical" value.
Average vs mean vs median vs mode
Mean is the sum ÷ count. Median is the middle value once sorted. Mode is the most frequent value. "Average" almost always means the mean. For 1, 2, 2, 9 the mean is 3.5, the median 2 and the mode 2.