Answer
See the explanation
Work Step by Step
Speed of the boat $x$
Speed of the current $y$
Speed of boat downstream $x+y$
Speed of boat upstream $x-y$
-Speed of boat downstream $\times$ time downstream $=$ distance downstream.
$(x+y)1=20$
-Speed of boat upstream $\times$ time upstream $=$ distance upstream.
$(x-y)\frac{5}{2}=20$
Therefore,
$\begin{cases}
x+y=20\\
\frac{5}{2}x-\frac{5}{2}y=20
\end{cases}$
Multiplying the second equation by 2.
$\begin{cases}
x+y=20\\
5x-5y=40
\end{cases}$
Multiplying the first equation by 5 and adding it together.
$\begin{cases}
5x+5y=100\\
5x-5y=40\\
-- -- --\\
10x=140
\end{cases}$
Thus, $x=14$. Substituting back in, $14+y=20, y=6$
