Ashu can row 7.5 km an hour in still water. He finds that it takes four times as much time to row upstream, as it takes to row downstream. The speed of the stream is:
Show answer & explanation
Correct answer: B
Explanation
The correct answer is Option 2 i.e. 4.5 km/h
Speed of Ashu in still water = 7.5 km/h
Let the speed of stream be x km/h
Let the time taken to row downstream be t hours.
Time taken to row upstream = 4t hours
Downstream speed = (7.5 + x)
Upstream speed = (7.5 - x)
Distance = Speed × Time
From Downstream -
⇒ Distance = (7.5 + x)t ...(1
From Upstream -
⇒ Distance = (7.5 - x)4t ...(2
Equating eq(1 and (2 -
⇒ (7.5 + x)t = (7.5 - x)4t
⇒ 7.5 + x = 30 - 4x
⇒ 5x = 30 - 7.5
x = 22.5/5 = 4.5 km/hr