Q. what does the mathematics mean means

The mean, or average, is the sum of a set of numbers divided by the  number of numbers in the set.  For example:

1+3+4+5=13 

The set consists of 4 numbers (1, 3, 4, 5); the sum of these numbers is 13.  The mean or average is the sum (13) divided by the number of numbers in the set (4) -- 13/4

13/4=3.25

The mean is 3.25.

Trying again..  Here's the set:  1 3 4 7 9  There are 5 numbers in this set.

Add the numbers together and divide that sum by the numbers in the set.