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A boat travel $240 \mathrm{~km}$ downstream in 3 hours and the time is taken by the boat to travel the same distance in upstream in 6 hours. Find the speed of stream (in kmph).
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The time is taken by the boat to cover upstream distance $=6$ hours

Formula used:
Upstream speed $=u-v$
Downstream speed $=u+v$
Where,
$\mathrm{u}$, is the speed of the boat
$v$, is the speed of the stream
Speed $=\frac{\text { Distance }}{\text { Time }}$

Calculation:
The upstream speed,
\begin{aligned} &\Rightarrow \mathrm{u}-\mathrm{v}=\frac{240}{6} \\ &\Rightarrow \mathrm{u}-\mathrm{v}=40 \mathrm{kmph} \quad-(1) \end{aligned}
The downstream speed,
\begin{aligned} &\Rightarrow \mathrm{u}+\mathrm{v}=\frac{240}{3} \\ &\Rightarrow \mathrm{u}+\mathrm{v}=80 \mathrm{kmph} \quad-(2) \end{aligned}
On subtracting the equation (1) from equation (2), we get,
\begin{aligned} &\Rightarrow 2 \mathrm{v}=40 \\ &\Rightarrow \mathrm{v}=20 \mathrm{kmph} \end{aligned}

The speed of the stream $=20 \mathrm{kmph}$

$\therefore$ The speed of the stream is $20 \mathrm{kmph}$.

by Diamond (44,867 points)

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