Answer
$\approx7.91$
Work Step by Step
The mean of $n$ numbers is the sum of the numbers divided by $n$.
Hence the mean: $\frac{5+10+15+20+25}{5}=15$
The standard deviation of $x_1,x_2,...,x_n$ is (where $\overline{x}$ is the mean of the data values): $\sqrt{\frac{(x_1-\overline{x})^2+(x_2-\overline{x})^2+...+(x_n-\overline{x})^2}{n-1}}$.
Hence here the the standard deviation is: $\sqrt{\frac{(5-15)^2+(10-15)^2+(15-15)^2+(20-15)^2+(25-15)^2+}{5-1}}\approx7.91$
