SSC CGL 20 Aug 2021 Shift 2 Paper

Let upstream speed $=\mathit{U}\mathit{k}\mathit{m}∕\mathit{h}$
Down stream speed $=\mathit{D}\mathit{k}\mathit{m}∕\mathit{h}$
According to the question,
$\frac{5}{\mathit{U}}+\frac{\frac{15}{2}}{\mathit{D}}=\frac{45}{60}$

and $\frac{2.5}{\mathit{U}}+\frac{5}{\mathit{D}}=\frac{25}{60}$ . . . (ii)
Eq. (ii) multiply by $2-$ Eq. (i),
$\Rightarrow \frac{5}{\mathit{U}}+\frac{10}{\mathit{D}}-\frac{5}{\mathit{U}}-\frac{7.5}{\mathit{D}}=\frac{50}{60}-\frac{45}{60}$
$\Rightarrow \frac{2.5}{\mathit{D}}=\frac{5}{60}$
$\Rightarrow \mathit{D}=30\mathit{k}\mathit{m}∕\mathit{h}$
Time will take to cover $6\mathit{k}\mathit{m}$ downstream.

$=\frac{1}{5}\mathit{k}\mathit{m}∕\mathit{h}$