Question:
A person can row a boat at the rate of 5 km/hour in still water. He takes thrice as much time in going 40 km upstream as in going 40 km downstream. Find the speed of the stream.
Solution:
Given that,
Speed of boat in still water = 5 km/hour.
Distance covered by boat = 40 km
Now we are assuming that the speed of the stream is
, then
Speed of the boat rowing upstream =
, and
Speed of the boat rowing downstream = 
According to the given condition, time taken to cover 
in upstream is three times the time taken to cover in downstream, therefore
$\frac{40}{5-v}=3 \times \frac{40}{5+v}$
$\Rightarrow 5+v=15-3 v$
$\Rightarrow \quad 4 v=10$
$\Rightarrow \quad v=2.5$
Hence the speed of stream is 2.5 km/hour.