Đáp án:

${t_3} = \dfrac{{2{t_1}{t_2}}}{{{t_1} + {t_2}}}$ 

Giải thích các bước giải:

Thời gian thuyền trôi là:
$\begin{array}{l}
\left\{ \begin{array}{l}
{t_1} = \dfrac{s}{{v + u}} \Leftrightarrow \dfrac{1}{{{t_1}}} = \dfrac{v}{s} + \dfrac{u}{s}\\
{t_2} = \dfrac{s}{{v - u}} \Leftrightarrow \dfrac{1}{{{t_2}}} = \dfrac{v}{s} - \dfrac{u}{s}\\
{t_3} = \dfrac{s}{u} \Leftrightarrow \dfrac{1}{{{t_3}}} = \dfrac{u}{s}
\end{array} \right. \Leftrightarrow \dfrac{1}{{{t_1}}} - \dfrac{1}{{{t_2}}} = \dfrac{v}{s} - \dfrac{v}{s} + \dfrac{u}{s} + \dfrac{u}{s}\\
 \Leftrightarrow \dfrac{2}{{{t_3}}} = \dfrac{1}{{{t_1}}} - \dfrac{1}{{{t_2}}} \Leftrightarrow \dfrac{2}{{{t_3}}} = \dfrac{{{t_2} - {t_1}}}{{{t_1}{t_2}}} \Leftrightarrow {t_3} = \dfrac{{2{t_1}{t_2}}}{{{t_2} - {t_1}}}
\end{array}$