Question

A boat can travel $36$ km upstream in $5$ hours. If the speed of the stream is $2.4$ km/hr, how much time will the boat take to cover a distance of $78$ km downstream?

• A. $4.5$ hours
• B. $5$ hours
• C. $6.5$ hours
• D. $6$ hours

Answer 1

The correct option is C $6.5$ hours

Distance covered by boat in $5$ hours $=36$ km
Speed of boat in upstream $=\frac{36}{5}=7.2$ km/h
Speed of the stream $=2.4$ km/hr
$\therefore$ Speed of the boat in still water $=(7.2+2.4)=9.6$ km/hr
Now, speed of boat in downstream $=(9.6+2.4)=12$ km/hr
$\therefore$ Time taken in covering $78$ km distance $=\frac{78}{12}=6.5$ hours