Question

a motorboat whose speed is 15 km per hour in still water takes more time to go 20 km upstream then to return downstream to same spot what is the average speed

Answer 1

Speed of motorboat = 15 km
Speed of stream = s km/hr
Speed of boat when it goes upstream = 15 - s
Speed of boat when it goes downstream = 15 + s

Time taken = distance travelled / speed 
Time taken to travel 20 km upstream  = 20/ (15 - s)
Time taken to travel 20 km downstream = 20/ (15+s)

{20/ (15 - s) - 20/ (15+ s)} = 1

Therefore the speed of the stream comes out to be 2.6 km/ hr.
 

Answer 2

Answer:

the answer is 2.6 km\hr

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