Ta có : ${{s}_{1}}={{s}_{2}}=\frac{s}{2}=0,5s$ , $t={{t}_{1}}+{{t}_{2}}=\frac{{{s}_{1}}}{{{v}_{1}}}+\frac{{{s}_{2}}}{{{v}_{2}}}$

Vận tốc trung bình trên cả quãng đường :

${{v}_{tb}}=\frac{s}{t}=\frac{s}{\frac{{{s}_{1}}}{{{v}_{1}}}+\frac{{{s}_{2}}}{{{v}_{2}}}}=\frac{s}{\frac{0,5s}{12}+\frac{0,5s}{{{v}_{2}}}}=\frac{s}{0,5s\left( \frac{1}{12}+\frac{1}{{{v}_{2}}} \right)}=\frac{2}{\left( \frac{1}{12}+\frac{1}{{{v}_{2}}} \right)}=8\,\,km/h$

$\Rightarrow \frac{1}{12}+\frac{1}{{{v}_{2}}}=\frac{2}{8}=\frac{1}{4}\Rightarrow \frac{1}{{{v}_{2}}}=\frac{1}{4}-\frac{1}{12}=\frac{1}{6}\Rightarrow {{v}_{2}}=6km/h$