Question
A man can go on a motor boat with a speed of 15 km/hour
in still water, he goes to a certain distance upstream 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 u km/hour and speed of current be v km/hour Average speed = 2xy/(x+y) (here, x is speed of downstream and y is speed of upstream) = (2 (u+v)(u-v))/((u+v+u-v)) = (u2-v2)/u = (225-25)/15 = 40/3 km/hour IXAMBEE Method :- 
Average speed = (Total distance covered)/(total time taken) = (2×60)/9 =40/3 km/hour
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