Chapter 3 - Section 3.2 - Assess Your Understanding - Vocabulary and Skill Building - Page 151: 10

The variance: $σ^2=25$ The standard deviation: $σ=5$

At first, let's evaluate the population mean: $μ=\frac{4+10+12+12+13+21}{6}=12$ Now, the variance: $σ^2=\frac{Σ(x_i-μ)^2}{n}=\frac{(4-12)^2+(10-12)^2+(12-12)^2+(12-12)^2+(13-12)^2+(21-12)^2}{6}=\frac{64+4+0+0+1+81}{6}=25$ The standard deviation: $σ=\sqrt {25}=5$