Chapter 3 - Section 3.2 - Assess Your Understanding - Applying the Concepts - Page 152: 20a

$σ=12.84~min$

First, the mean: $μ=\frac{39+21+9+32+30+45+11+12+39}{9}=26.44$ Now, the standard deviation: $σ=\sqrt {\frac{Σ(x_i-μ)^2}{n}}=\sqrt {\frac{(39-26.44)^2+(21-26.44)^2+(9-26.44)^2+(32-26.44)^2+(30-26.44)^2+(45-26.44)^2+(11-26.44)^2+(12-26.44)^2+(39-26.44)^2}{9}}=12.84$