\[1)\ v = \frac{\mathrm{\Delta}S}{\mathrm{\Delta}t} = \frac{150 - 0}{2 - 0} =\]
\[= \frac{150}{2} = 75\ \left( км\text{/}ч \right).\]
\[Ответ:\ \ 75\ км\text{/}ч.\]
\[2)\ t = \mathrm{\Delta}t_{1} + \mathrm{\Delta}t_{2} =\]
\[= (5 - 2) + (8 - 6) =\]
\[= 3 + 2 = 5\ ч.\]
\[Ответ:\ \ 5\ ч.\]
\[3)\ v = \frac{\mathrm{\Delta}S_{1} + \mathrm{\Delta}S_{2} + \mathrm{\Delta}S_{3}}{\mathrm{\Delta}t_{1} + \mathrm{\Delta}t_{2} + \mathrm{\Delta}t_{3}} =\]
\[= \frac{(150 - 0) + (250 - 150) + (250 - 0)}{(2 - 0) + (6 - 5) + (12 - 8)} =\]
\[v = \frac{150 + 100 + 250}{2 + 1 + 4} = \frac{500}{7} =\]
\[= 71\frac{3}{7} \approx 71,4\ км\text{/}ч.\]
\[Ответ:\ \approx 71,4\ км\text{/}ч.\]