Rajesh rows in still water with a speed of 4.5 kmph to go to a certain place and comes back. Find his average speed for the whole journey, if the river is flowing with a speed of 1.5 kmph?

Let the distance in one direction = k kms

Speed in still water = 4.5 kmph

Speed of river = 1.5

Hence, speed in upstream = 4.5 - 1.5 = 3 kmph

Speed in downstream = 4.5 + 1.5 = 6 kmph

Time taken by Rajesh to row upwards = k/3 hrs

Time taken by Rajesh to row downwards = k/6 hrs

Now, required Average speed = $\frac{\mathrm{Total} \mathrm{distance}}{\mathrm{Total} \mathrm{speed}} = \frac{2k}{\frac{k}{3}} = \frac{2k x 18}{6k + 3k} = 4 \mathrm{kmph}$ 

Therefore, the average speed of the whole journey = 4kmph.