The speed of a boat in still water is 5 km/hr. If it takes thrice as much time in going 20 km upstream as in going the same distance downstream, find the speed of the stream.

Given, speed of a boat in still water (V) =  5 km/hr

Let speed of the stream be V' km/hr.

time taken to go 20km downstream (t) = distance travelled$$\div$$relative speed of the boat 

= $$\frac{20}{V + V'}$$

=$$\frac{20}{5 + V'}$$ hours

time taken to go 20 km upstream (t') =  distance travelled$$\div$$relative speed of the boat 

= $$\frac{20}{V - V'}$$

= $$\frac{20}{5 - V'}$$ hours

Given that t' = 3*t

$$\Rightarrow \frac{20}{5 - V'} = 3\times \frac{20}{5 + V'}$$

$$\Rightarrow 4V' = 10$$

$$\Rightarrow V' = 2.5$$ km/hr.

So the speed of the stream is 2.5 km/hr.