Mean
The mean is an average of at least two numbers. To determine this, all values are added and divided by the number.
The general formula for calculating the mean is:
$\frac{x_1+x_2+\dots+x_n}{n}$
$x$ ... values
$n$ ... number of values
Example
Peter took part in a flea market on 4 different days and earned 15 €, 25 €, 9 € and 18 €. Calculate how much Peter earned on average in one day.
$\frac{15+25+9+18}{4}=\frac{67}{4}=16.75$
Peter earned an average of 16.75 € a day.