A train travels from city A to city B at a speed of 60 mph and returns from city B to city A at a speed of 40 mph. What is the average speed of the train for the entire trip?
Correct: A
Let the distance between city A and city B be 'd'. Time taken to travel from A to B = d/60. Time taken to travel from B to A = d/40. Total distance = 2d. Total time = d/60 + d/40 = (2d + 3d)/120 = 5d/120 = d/24. Average speed = Total distance / Total time = 2d / (d/24) = 2d * (24/d) = 48 mph.