A person can row a boat at 5 km/hr in still water. If the speed of water current in a river is 1 km/hr, and it takes him 1 hour to row to a place and come back, how far off is the place?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2.4 km
$\text{Speed in still water}=5\text{ km/hr}$
$\text{Speed of current}=1\text{ km/hr}$
$\text{Downstream speed}=6\text{ km/hr},\quad \text{Upstream speed}=4\text{ km/hr}$
$\frac{d}{6}+\frac{d}{4}=1$
$d\left(\frac{1}{6}+\frac{1}{4}\right)=1$
$d\left(\frac{2+3}{12}\right)=1$
$d\cdot\frac{5}{12}=1$
$d=\frac{12}{5}$
The distance of the place is $\frac{12}{5}$ km.