Bunuel wrote:
A man can row at a speed of 5 km/hr in still water. If he rows a certain distance upstream and back to the starting point in a river which flows at 1.5 km/hr, what is his average speed for the double journey?
A. 3.5 km/h
B. 4.55 km/h
C. 5 km/h
D. 5.5 km/h
E. 6.5 km/h
Let the distance traveled upstream be D
We know, Speed= Distance / Time
Time for traveling upstream = Distance/Speed = D/ 5-1.5 = D /3.5
Distance traveled upstream be D
Time for traveling upstream = Distance/Speed = D/ 5+1.5 = D /6.5
Total Time = D /3.5 + D /6.5
Average Speed = Total Distance/ Total Time = D + D / (D /3.5 + D /6.5 ) = (2D * 7 * 5 * 13) / (200 D) 4.55