A person can row a boat at the rate of 5 km/hour in still water. He 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