Answer
$\approx6.32$
Work Step by Step
The mean of $n$ numbers is the sum of the numbers divided by $n$.
Hence the mean: $\frac{4+8+12+16+20}{5}=12$
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: $\sqrt{\frac{(4-12)^2+(8-12)^2+(12-12)^2+(16-12)^2+(20-12)^2}{5-1}}\approx6.32$
