Chapter 3 - Section 3.2 - Assess Your Understanding - Applying the Concepts - Page 152: 19a

$σ=7.671~beats~per~minute$

First, the mean: $μ=\frac{76+60+60+81+72+80+80+68+73}{9}=72.22$ Now, the standard deviation: $σ=\sqrt {\frac{Σ(x_i-μ)^2}{n}}=\sqrt {\frac{(76-72.22)^2+(60-72.22)^2+(60-72.22)^2+(81-72.22)^2+(72-72.22)^2+(80-72.22)^2+(80-72.22)^2+(68-72.22)^2+(73-72.22)^2}{9}}=7.671$