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    Sunday, January 31, 2016

    Calculation of Boat speed in Upstream and Down stream water using simple formulas

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    Definitions:
    In water, the direction along the stream is called downstream.
    direction against the stream is called upstream.

    If the speed of a boat in still water is a km/hr and the speed of the stream is b km/hr, then:

    Speed downstream = (a + b) km/hr.
    Speed upstream = (a- b) km/hr.

    If the speed downstream is a km/hr and the speed upstream is b km/hr, then:

    Speed in still water = 1/2(a + b) km/hr.
     Rate of stream = 1/2(a - b) km/hr.

    Examples 1: If a boat travels 8 km upstream in 24 minutes and speed of stream in 4 kmph. Then what is the speed of boat in still water?
     A. 26kmph  
     B.22 kmph
     C.24 kmph
     D.29 kmph

    Solution: As you know of boat  travles 8 km in 24 minutes, you can find the upstream speed 

    8 km - 24 min
    ? km - 60 min 
    = (60/24)X8=20 km=20km/hr=20kmph

    Still water speed=upstream speed+speed of the stream(by modifying above forumula Speed upstream = (a- b) km/hr.)
    =20+4
    =24 kmph

    Correct answer is C

    Example 2: A boat can travel with speed of 24 km/hr ins still water. Speed of the stream is 4 km/hr. Find the time taken by boat to travel 68 km upstream
     A) 4.3 hrs B) 3.4 hrs C) 3.8 hrs D) 4.4 hrs 

    Solution:  We have to first find out upstream speed in order to calculate time. 
                 upstream speed=still water speed-stream speed
    = 24-4=20.
    So boat travels with 20 kmph upstream. which means 20 km in 1  hour
    20 km  - 60 min
    68 km -  ?
    =(68/20)X60=204 min
    =204/60=3.4 hrs

    So the correct answer is B


    Have any doubts ? comment below we will reply within hour.







       

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