A motor boat whose speed is 18 km/h in still water takes 1 hour

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255. A motor boat whose speed is 18 km/h in still water takes 1 hour more to go 24 km upstream than to return downstream to the same spot. Find the speed of the stream. 


Let the speed of the stream be x km/hr.
∴ Speed of the boat upstream = (18 – x) km/hr.
Speed of the boat downstream = (18 + x) km/hr.
Time taken for going 24 km upstream = fraction numerator 24 over denominator 18 minus straight x end fraction space hours

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According to given condition,
∴        space fraction numerator 24 over denominator 18 minus straight x end fraction minus fraction numerator 24 over denominator 18 plus straight x end fraction equals 1

rightwards double arrow space space 24 left parenthesis 18 plus straight x right parenthesis space minus space 24 left parenthesis 18 minus straight x right parenthesis space equals space left parenthesis 18 minus straight x right parenthesis left parenthesis 18 plus straight x right parenthesis
rightwards double arrow space space space space space space space space space space space space space space straight x squared plus 48 straight x minus 324 space equals space 0
Using the quadratic formula, we get
straight x equals fraction numerator negative 48 plus-or-minus square root of 48 squared plus 1926 end root over denominator 2 end fraction equals fraction numerator negative 48 plus-or-minus square root of 3600 over denominator 2 end fraction
equals space fraction numerator negative 48 plus-or-minus 60 over denominator 2 end fraction equals 6 space or space minus 54
Since x is the speed of the stream, it cannot be negative. So, we ignore the root x = –54. Therfore, x = 6 gives the speed of the stream as 6km/h.
rightwards double arrow space space space space space space space 21 over 4 cross times fraction numerator 2 straight x over denominator 25 minus straight x squared end fraction equals 1
rightwards double arrow space space space space space space space space space 21 over 2 cross times fraction numerator straight x over denominator 25 minus straight x squared end fraction equals 1
rightwards double arrow space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space 21 straight x space equals space 50 minus 2 straight x squared
rightwards double arrow space space space space space space space space space space space space space 2 straight x squared plus 21 straight x minus 50 space equals space 0
rightwards double arrow space space space space 2 straight x squared plus 25 straight x minus 4 straight x minus 50 space equals space 0
rightwards double arrow space space space space straight x left parenthesis 2 straight x plus 25 right parenthesis minus 2 left parenthesis 2 straight x plus 25 right parenthesis space equals space 0
rightwards double arrow space space space space space space space space space space space space space left parenthesis 2 straight x plus 25 right parenthesis space left parenthesis straight x minus 2 right parenthesis space equals space 0
rightwards double arrow space space 2 straight x space plus 25 space equals 0 space space space space space space straight x space minus space 2 space equals 0
rightwards double arrow space space space space space 2 straight x space equals space minus 25 space space space space space rightwards double arrow space space straight x space equals space 2
rightwards double arrow space space space space space straight x space equals space fraction numerator negative 25 over denominator 2 end fraction
straight x equals fraction numerator negative 25 over denominator 2 end fraction  is rejected.
Hence, the speed of the stream is 2 km/hr.

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