David can row a boat in still water at the rate of 5 km/hr. He rowed in a river downstream to meet his friend. After returning back he observed that the duration of the upstream journey was three times that of the downstream journey. The speed of the stream was:

2.5 km/hr

The correct answer is Option (2) → 2.5 km/hr

Speed of David in still water = $5\, km/hr$

Speed of Stream = $s\, km/hr$

Downstream speed = $5 + s\, km/hr$

Upstream speed = $5 - s\, km/hr$

Distance traveled one way = $d\, km$

Time taken to row downstream = $t$ hours

Time upstream = 3 × Time downstream

$\frac{d}{5-s}=3 ×\frac{d}{5+s}$

$⇒5+s=3(5-s)$

$⇒5+s=15-3s$

$⇒4s=10$

$⇒s=2.5km/hr$