Question
A man can go on a motorboat with a speed of 18 km/hour
in still water to a certain upstream point and return to the starting point in a river that flows at a speed of 14 km/hour. Find the average speed of the motorboat for the total journey.Solution
Let the speed of the boat be b km/hour and the speed of the current be s km/hour. Average speed = 2xy/ (x+ y) (here, x is the speed of downstream and y is the speed of upstream)  = (2 (b +s) (b-s))/ ((b+ s +b -s)) = (b2 – s2)/b = (324 - 196)/18  = 128/18  = 64/9 Km/h
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