Sohan covers four trips for same distance. His speed on first trip was 240 km/hr and in each subsequent trip his speed was half of the previous trip. What is the average speed of Sohan for these four trips?

64 km/hr 

Let the distance covered in each trip is d,

For first trip, time = $\frac{distance}{speed}$

                          = $\frac{d}{240}$

For second trip, time = $\frac{distance}{speed}$

                              = $\frac{d}{120}$

For third trip, time = $\frac{distance}{speed}$

                           = $\frac{d}{60}$

For fourth trip, time = $\frac{distance}{speed}$

                             = $\frac{d}{30}$

Average speed = $\frac{total\; distance}{total\; time}$

                       = $\frac{4d}{\frac{d}{240} + \frac{d}{120} + \frac{d}{60} + \frac{d}{30}}$

                        = $\frac{4d}{\frac{15d}{240}}$

                         =  64 km/hr