Question

The speed of the boat in still water is 11 km/ hr. It can go 21 km upstream and 12 km downstream in 3 hours. 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 11-S (Against the water flow) and downstream will be S+ l 1 (Towards the water flow and hence speed is added). 

Distance travelled = 21 Km Upstream and 12 km downstream 

Total Time = 3 hours 

Time = Distance / Speed. 

`21/(11 - "S") + 12/(11 + "S") = 3`

⇒ 21(11+S) + 12 (11-S) = 3(11+S)(11-S) 

⇒ 231 + 21S + 132 - 12S = -3S² + 363 

⇒ 3S² + 9 s = 0 

⇒ 3S(S+3)=0 

⇒ S=O ,-3 

S = O

Since, speed cannot be negative, thus, the speed of the stream is 0 km/ hr.