You drive 4 km at 30 km/h and then another 4 km at 50 km/h what is the average speed

Average speed = Distance traveled / time taken

In first case Time taken = distance/speed

                                        = 4/30 = 0.1333 hours

In second case Time taken = distance/speed

                                        = 4/50 = 0.08 hours

So total time taken , t = 0.1333+0.08 = 0.2133 hours

Total distance traveled = 4 km + 4 km = 8 km

So average speed = 8/0.2133 = 37.50 km/hr

Average speed of bus = 10.42 m/s

So the average speed = 10.42 m/s or 37.50 km/hr.