The speed of a motorboat in still water is 20 km/h. It travels 150 km downstream and then returns to the starting point. If the round trip takes a total of 16 hours, what is the speed (in km/h ) of the flow of the river?
Show answer & explanation
Correct answer: C
Explanation
The correct answer is option 3 i.e. 5.
The boat takes 16 hours to go 150 km downstream and 150 km upstream.
Speed of the motorboat in the still water = 20 km/h
When going downstream, the speed of the boat and the flow of the river are added to calculate the net speed while going upstream, we take the difference between the speed of the boat and the flow of the river as the net speed.
Time = Distance / Speed
Let the flow of the river be x km/h
According to the question -
⇒ {150/(20 + x)} + {150/(20 - x)} = 16
⇒ {150 × (20 + x + 20 - x)}/(202 - x2) = 16
⇒ (150 × 40)/(202 - x2) = 16
⇒ 400 - x2 = 375
⇒ x2 = 25
⇒ x = 5 km/h