Question
When travelling with the current, a man takes 4 hours to cover the distance that he would've covered in 12 hours while travelling against the current. If the current flows at 10 km/h, then find the time taken by the boat to cover 10 km in still water.
Solution
Let the speed of the boat in still water be 'x' km/hr Therefore, distance covered in upstream = 12 X (x - 10) km Also, distance covered in downstream = 4 X (x + 10) km According to the question, 12 X (x - 10) = 4 X (x + 10) Or, {(x + 10)/(x - 10)} = (12/4) = (3/1) Or, x + 10 = 3x - 30 Or, 2x = 40 => 'x' = 20 So, speed of the boat in still water = 20 km/h So, required time = (10/20) X 60 = 30 minutes
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