Question

The speed of a boat in still water is 15km/ hr. It can go 30km upstream and return downstream to the original point in 4 hours 30 minutes. Find the speed of the stream.

Answer 1

Let the speed of the stream be S km/ hr. So in upstream, boat speed will be 15-S (Against the water flow) and downstream will be S+15 (Towards the water flow and hence speed is added). 

Distance travelled is same= 30 KM . Total Time = 4.5 hours 

Time = Distance / Speed.

`30/(15 - "S") + 30/(15 + "S") = 4.5`

⇒  30 x (15+S) + 30 x (15-S) = 4.5 x (15+S) x (15-S) 

⇒  200= 225-S² 

⇒  S² = 25 

Hences = 5 Km / hr.