Question

A boat takes $4$ hours to move $10km$ and back to starting point in still water. If the river flow velocity is $2km/hr$, then time taken by the boat to move $10km$ upstream and back to starting point is approximately

• A. $4hours$
• B. $5hours$
• C. $6hours$
• D. $7hours$

Answer 1

The correct option is B $5hours$

Speed of boat in still water $=\frac{10+10}{4}=5km/hr$.

Time taken by boat to cover $10km$ and come back to starting point

$=$ Time taken to go upstream + Time taken to come downstream.

$=\frac{10}{5-2}+\frac{10}{5+2}=\frac{100}{21}=4.77\approx 5hr.$