\begin{gather*} 3 = (1-r) \times 2 + r \times 5\\ \Rightarrow r = \frac{1}{3}\\ \Rightarrow (n+1)s = 1 + \frac{1}{3} = \frac{4}{3}\\ \Rightarrow s = \frac{4}{15} \approx 0.267 \end{gather*}