Ricky can go on a motor boat with a speed of 15 km/hour in still water to a certain upstream point and come back to the starting point in a river which flows at a speed of 5km/hour. Find the average speed of motor boat for total journey?
Let, the speed of boat be b km/hour and speed of current be s km/hour Average speed = 2xy/(x+y) (here, x is speed of downstream and y is speed of upstream) = (2 (b+s)(b-s))/((b+s+b-s)) = (b2-s2)/b = (225-25)/15 = `13 1/3` km/hour
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