A person, rowing at the rate of 5km/h in still water, takes thrice as much time in going 40 km upstream as in going 40 km downstream. Find the speed of the stream.

Explanation

Let the speed of stream = x km/h
Given that speed of person rowing in still water = 5 km/h
`therefore` Speed of person rowing in upstream = (5 - x) km/h
and speed of person rowing in downstream = (5 + x) km/h
Now time taken in 40 km downstream,
`t_(1) = (40)/(5 + x) h (because "time = "("distance")/("speed"))`
and time taken in 40 km upstream
`t_(2) = (40)/(5 - x)h`
Given that `t_(2) = 3t_(1)`
implies `(40)/(5-x) = (3 xx 40)/(5 + x)`
implies 5 + x = 15 - 3x
implies 4x = 10
implies x = 2.5 km/h
`therefore` Speed of stream = 2.5 km/h