Question

A motor in still water travels at a speed of $3.6$ km/hr. It goes $56$ km upstream in $1$ hour $45$ min. The time taken by it to cover the same distance downstream is:

• A. $2$ hours $25$ min
• B. $3$ hours
• C. $1$ hour $24$ min
• D. $2$ hours $21$ min

Answer 1

The correct option is C $1$ hour $24$ min

Let the speed of the stream be $x$ km/hr.
Then, $\frac{56}{\frac{7}{4}}=36-x$

$\Rightarrow$ $\frac{56\times 4}{7}=36-x$

$\Rightarrow x=36-32=4$ km/hr

$\therefore$ Time taken by it to cover the same distance

Upstream $=$ $\frac{56}{36+4}$ hrs

$=\frac{56}{40}$ hrs

$=$ $1\frac{2}{5}$ hours

$=1$ hour $24$ min