CMAT 2019 Solved Question Paper

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 relative speed of the boat
$=\frac{20}{\mathit{V}+\mathit{V}\prime }=\frac{20}{5+\mathit{V}\prime }\mathrm{hours}$
time taken to go 20 km upstream (t') = distance travelled ÷ relative speed of the boat
$=\frac{20}{\mathit{V}-\mathit{V}\prime }=\frac{20}{5-\mathit{V}\prime }\mathrm{hours}$
Given that $\mathit{t}\prime =3\times \mathit{t}$
$\Rightarrow \frac{20}{5-\mathit{V}\prime }=3\times \frac{20}{5+\mathit{V}\prime }$
$\Rightarrow 4\mathit{V}\prime =10$
$\Rightarrow \mathit{V}\prime =2.5$km/hr.
So the speed of the stream is 2.5 km/hr.