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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.
Solution
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(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.
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