If a car travels from city A to city B at an average speed of 60 km/h and returns at an average speed of 40 km/h, what is its average speed for the entire trip?
Correct: A
The average speed for the entire trip is given by the total distance divided by the total time. Let the distance from city A to city B be x. Then, the total distance is 2x. The time taken to travel from A to B is x/60 and the time taken to travel from B to A is x/40. Therefore, the total time is x/60 + x/40. The average speed is then given by 2x / (x/60 + x/40) = 2 / (1/60 + 1/40) = 2 / (4/240 + 6/240) = 2 / (10/240) = 2 * 240 / 10 = 48 km/h.