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