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CBSE Class 10 Answered

A motorboat whose speed is 9km/hr in still water,goes 15km downstream and comes back in a total time of 3hours45minutes.find the speed of the stream?
Asked by aditya95 | 12 Aug, 2010, 06:22: PM
answered-by-expert Expert Answer
T = time for downstream, t = time for upstream travel.
T + t = 3 hrs 45 min = 15/4 hrs.
For downstream, distance/velocity = time,
15/(V + v) = T
15/(9 + v) = T       ... (1)
Similarly,
15/(9 -  v) = t    ... (2)     ...where V - Speed of boat, v - speed of stream.
Add (1) and (2)
15/(9 + v) + 15/(9 -  v) = T + t
15(18)/(81 - v2)  = 15/4
v2 = 81 - (18x4) = 9
v = sped of stream is 3 km/hr.
Regards,
Team,
TopperLearning.
Answered by | 12 Aug, 2010, 10:46: PM

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