Question

A motorboat in still water is 9km/h goes 15 km downstream and comes back to the same spot in a total time of 3 hours 45 min.Find the speed of the stream

Answer 1

given

speed of the boat = 9km/h

let the speed of stream be x km/hr

then the speed of the boat upstream and downstream will be

(9-x)km/hr and (9+x) km/hr

the boat goes and comes 12km each in hours

=> 12/9+x +12/9-x= 3

=> 4/9+x +4/9-x =1

=> 4(9-x)+4(9+x) = (9+x)(9-x)

=> 36-4x +36+4x =81-

=> x^{2} =81-72=9

=> x= √9

=> x= 3

The speed of the stream 3km/hr

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