The formula
[tex]\bar{x}=\frac{1}{n}\sum_{j=1}^{n}x_j[/tex]
can be interpreted as
the mean of x is calculated as the sum of all n numbers from x1 to xn, divided by n.
Here we are given a list of nine numbers, so n=9 is the number of pieces of data required to calculate the mean. So the value of n=nine,
because the mean is calculated by the sum of nine numbers, and dividing the sum by 9.
