Q6 of 227 Page 107

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

Let x km/hr be the speed of water


Speed of boat is 15km/hr.


So, speed in upstream = (15 + x) km/hr.


speed in downstream = (15 – x) km/hr.


Let T1 be the time taken to cover the distance 30 km in upstream.


Let T2 be the time taken to cover the distance 30 km in downstream.





T1 + T2 = 4hours 30minutes



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⇒ 900 × 2 = 9(225 – x2)


Now, let us divide the entire equation by 9.


So, that we will get,


200 = 225 – x2


200 + x2 = 225


x2 = 225 – 200


x2 = 25


x = √25


x = ± 5


Speed must be positive so x =5 is the requied speed.


Speed of water = 5km/hr.


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