Time taken for the first part = 60/40 = 1.5 hours, Time taken for the second part = 90/60 = 1.5 hours, Total distance = 60 + 90 = 150 km, Total time = 1.5 + 1.5 = 3 hours, Average speed = Total distance / Total time = 150/3 = 50 km/h. Answer: b) 50 km/h.