Đáp án:

\(\left\{ \begin{array}{l}{v_1} = 28,8km/h\\{v_2} = 14,4km/h\end{array} \right.\)

Giải thích các bước giải:

Ta có: $s = 4,8km$

+ Nửa quãng đường đầu: \(\left\{ \begin{array}{l}{s_1} = 2,4km\\{t_1} = \dfrac{{{s_1}}}{{{v_1}}}\\{v_1} = \dfrac{{{s_1}}}{{{t_1}}}\end{array} \right.\)

+ Nửa quãng đường sau: \(\left\{ \begin{array}{l}{s_2} = 2,4km\\{t_2} = \dfrac{{{s_2}}}{{{v_2}}}\\{v_2} = \dfrac{{{s_2}}}{{{t_2}}} = \dfrac{{{v_1}}}{2}\end{array} \right.\)

Theo đầu bài, ta có: ${t_1} + {t_2} = 15p = \dfrac{1}{4}h$

\(\begin{array}{l} \Leftrightarrow \dfrac{{{s_1}}}{{{v_1}}} + \dfrac{{{s_2}}}{{{v_2}}} = \dfrac{1}{4}\\ \Leftrightarrow \dfrac{{2,4}}{{{v_1}}} + \dfrac{{2,4}}{{\dfrac{{{v_1}}}{2}}} = \dfrac{1}{4}\\ \Rightarrow {v_1} = 28,8km/h\end{array}\)  

\( \Rightarrow {v_2} = 14,4km/h\)