Answer
$3$
Work Step by Step
The mean of $n$ numbers is the sum of the numbers divided by $n$.
Hence the mean: $\frac{1+1+1+4+7+7+7}{7}=4$
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: $\sqrt{\frac{(1-4)^2+(1-4)^2+(1-4)^2+(4-4)^2+(7-4)^2+(7-4)^2+(7-4)^2}{7-1}}=3$
