The value of x⁻, is the mean of the x values of the table.
The mean of x is obtained by adding all values of x, and divided the result by the total number of data, which is 5.
Then, you have:
[tex]\bar{x}=\frac{8+9+10+11+12}{5}=\frac{50}{5}=10[/tex]Hence, the mean of x, which is used in the formula to calculate the r-value, is 10
