Question
A man 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?Solution
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 = 200/15 = (40/3) km/hour Alternate Method :- 
Average speed = (Total distance covered)/(total time taken) = (2×60)/9 = 120 /9 = 40/3 kmph
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