MCQ
A man goes $10\,km$ downstream in $2\,hrs$ and $30\,km$ upstream in $10\,hrs$ . How much time will be take to swim $40\,km$ in still water........$hrs$
- A$8$
- ✓$10$
- C$12$
- D$6$
✓
Answer
Correct option: B.
$10$
b
$\frac{10}{\mathrm{v}_{\mathrm{m}}+\mathrm{v}_{\mathrm{r}}}=2$
$\frac{10}{\mathrm{v}_{\mathrm{m}}+\mathrm{v}_{\mathrm{r}}}=2$
${\Rightarrow \mathrm{v}_{\mathrm{m}}+\mathrm{v}_{\mathrm{r}}=5}$ $...(i)$
${\frac{30}{\mathrm{v}_{\mathrm{m}}-\mathrm{v}_{\mathrm{r}}}=10}$
$\Rightarrow \mathrm{v}_{\mathrm{m}}-\mathrm{v}_{\mathrm{r}}=3$ $...(ii)$
From $( i )$ $\&$ $(ii)$
$\mathrm{v}=4 \mathrm{m} / \mathrm{s}$
$\mathrm{So}$ $\mathrm{t}=\frac{40}{4} =10 \mathrm{hrs}$
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