Add up all the values in the set and then divide by the number of items in the set.

For example, the average value of 15 and 6 and 12 is

30 + 12 + 24 divided by 3 which equals 66/3 = 22

To find the average number of days in a month.

Add up the total days for each month in the year and divide by the number of months in the year.

365 days divided by 12 months = 30.416 days in an average month.