Final answer:
To average 100km/hr for the entire trip, the train must travel the second 120km at 150 km/hr, which is option b.
Step-by-step explanation:
The question is asking at what speed the passenger train must travel the second 120km of its trip to average 100km/hr for the entire 240km trip. The first half of the trip is covered at 75km/hr, which will take 1.6 hours (120km / 75km/hr). To calculate the average speed of 100km/hr for the entire trip, the total time taken cannot exceed 2.4 hours (240km / 100km/hr). Subtracting the time taken for the first half of the trip from the total allowed time, we have 2.4 hours - 1.6 hours = 0.8 hours for the second half of the trip. Since the distance remains the same for the second half, the speed required is 120km / 0.8 hours = 150 km/hr.