The speed of a motor boat in still water is 14.4 times the speed of the current of water. If the motor boat covers a certain distance upstream in 6 hours 25 minutes, then the time taken by the motor boat to come back is :
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 5 hours 35 minutes
Let the speed of the current be c. Then, the speed of the water is 14.4 c.
Upstream speed = $14.4 c - c=13.4c$
Downstream speed = $14.4c+c=15.4c$
The boat covers a certain distance d upstream in 6 hour 25 min,
$d=13.4c×\left(\frac{72}{12}hr\right)$
for the downstream journey,
$T_{down}=\frac{d}{downstream\,speed}=\frac{13.4c×\frac{72}{12}}{15.4c}$
$=\frac{67}{77}×\frac{72}{12}=\frac{67}{12}hrs$
= 5 hours 35 minutes