A man goes to a place on bicycle at speed of 16 km/hr and comes back at lower speed. If the average speed is 6.4 km/hr in total, then the return speed (in km/hr) is :

$$\eqalign{ & {\text{Average speed :}} \cr & {\text{ = }}\frac{{2xy}}{{x + y}} \cr & = \frac{{2 \times 16 \times y}}{{16 + y}} \cr & \Rightarrow 6.4 = \frac{{32y}}{{16 + y}} \cr & \Rightarrow 102.4 + 6.4 = 32y \cr & \Rightarrow 102.4 = 25.6y \cr & \Rightarrow y = \frac{{102.4}}{{25.6}} \cr & \Rightarrow y = 4{\text{ km/hr}} \cr} $$