Question
Time taken by a boat to cover 180 km in downstream is
the same as to cover 140 km in upstream. Find the time taken by the boat to cover 360 km in downstream, if the speed of the boat in still water is 40 km/h.Solution
ATQ, Let the speed of the stream be 'x' km/h. ATQ: We know that, Downstream speed of a boat = Speed of the boat in still water + speed of the stream Upstream speed of a boat = Speed of the boat in still water - speed of the stream 
9(40−x) = 7(40+x) 360−9x = 280+7x 16x = 80 x = 80/16 x = 5 Therefore, speed of the stream = 5 km/h Therefore, time taken to cover 360 km in downstream = 360/(40+5) = 360/45 = 8 hrs
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