A person can row  km an hour in still water and he finds that it takes him twice as long to row up as to row down the stream. The speed of the stream is:

Given:

Speed in still water (u) = 22.5 km/hr

Time taken to row upstream = 2 × Time taken to row downstream

Formula used:

Speed downstream = (u + v) km/hr

Speed upstream = (u - v) km/hr

Where, v = speed of stream

Calculation:

Let the speed of stream be v km/hr.

Time taken to row downstream = d / (u + v)

Time taken to row upstream = d / (u - v)

Given, Time taken to row upstream = 2 × Time taken to row downstream

⇒ d / (u - v) = 2 × d / (u + v)

⇒ 1 / (u - v) = 2 / (u + v)

⇒ u + v = 2 × (u - v)

⇒ u + v = 2u - 2v

⇒ 3v = u

⇒ v = u / 3

Given, u = 22.5 km/hr

⇒ v = 22.5 / 3

⇒ v = 7.5 km/hr

∴ The correct answer is option 1.