A man travelled a certain distance train at the rate of 25 kmph and walked back at the rate of 4 kmph. If the whole journey took 5 hours 48 minutes, the distance was :

$$\eqalign{ & {{\text{S}}_{{\text{average}}}} = \frac{{2ab}}{{a + b}} \cr & = \frac{{2 \times 25 \times 4}}{{25 + 4}} \cr & = \frac{{200}}{{29}}{\text{ km/hr}} \cr & {\text{Now, 2D = }}\frac{{200}}{{29}} \times \left( {5 + \frac{4}{5}} \right) \cr & = \frac{{200}}{{29}} \times \frac{{29}}{5} \cr & = 40{\text{ km}} \cr & {\text{ = D = 20 km}} \cr} $$