$S_1 = \frac{1}{3}S = \frac{S}{3}$
$⇔ t_1 = \frac{S_1}{v_1} = \frac{\frac{S}{3}}{v_1}= \frac{S}{3v_1}$
$S_2= \frac{1}{3}S = \frac{S}{3}$
$⇔t_2 = \frac{S_2}{v_2} = \frac{\frac{S}{3}}{v_2} = \frac{S}{3v_2}$
$S_3 = \frac{1}{3}S = \frac{S}{3}$
$⇔t_3=\frac{S_3}{v_3} = \frac{\frac{S}{3}}{v_3}= \frac{S}{3v_3}$
$v_{tb}=\frac{S_1 + S_2 +S_3}{t_1+ t_2 + t_3}= \frac{S}{\frac{S}{3v_1}+\frac{S}{3v_2}+\frac{S}{3v_3}}$
$⇔\frac{1}{\frac{1}{3v_1}+\frac{1}{3v_2}+\frac{1}{3v_3}}$
$⇒ v_{tb}= \frac{1}{\frac{1}{3.15}+\frac{1}{3.10}+\frac{1}{3.5}}≈8,2 km/h $