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

15 + 6 + 12 divided by 3 which equals 33/3 = 11

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.

Average values can look a bit silly. The average family is 2 parents and 1.5 babies.