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Proceed as follow:
the mean is given by:
[tex]\bar{x}=\frac{13+7+13+8+9}{5}=10[/tex]that is, the sum of all data over the number of data.
the standard deviation is:
[tex]\begin{gathered} s=\sqrt[]{\frac{\Sigma(x-\bar{x})^2}{n-1}} \\ s=\sqrt[]{\frac{(13-10)^2+(7-10)^2+(13-10)^2+(8-10)^2+(9-10)^2}{5-1}} \\ s=\sqrt[]{\frac{32}{4}}=\sqrt[]{8}\approx2.8 \end{gathered}[/tex]the variance is:
[tex]s^2=8[/tex]