Question
A boat travels a certain distance downstream in 72
minutes but takes 3 hours to cover the same distance upstream. The speed of the stream is 15 km/h. If the boat's speed in still water were reduced by 20%, how much time (in minutes) would it take for the boat to travel 236.5 km downstream?Solution
Let the speed of boat in still water be 'x' km/h, respectively. ATQ, D = (x + 15) X 1.2 = 1.2x + 18 .......(i) Also, D = (x - 15) X 3 = 3x - 45 ........(ii) Solving equations (i) and (ii) , we get, 1.2x + 18 = 3x - 45 Or, 1.8x = 65 So, 'x' = (63/1.8) = 35 Therefore, required time = 236.5 ÷ (0.8 X 35 + 15) = (236.5/43) = 5.5 hour = 5.5 X 60 = 330 minutes
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