A car travels from city A to city B at an average speed of 50 km/h and returns at an average speed of 60 km/h. What is the average speed for the round trip?
Correct: D
Let's denote the distance from A to B as D. The time to travel from A to B is D / 50, and the time to travel back is D / 60. The total time for the round trip is D / 50 + D / 60. To add these fractions, find a common denominator: (6D + 5D) / 300 = 11D / 300. The total distance for the round trip is 2D. The average speed = Total Distance / Total Time = 2D / (11D / 300) = 2 * 300 / 11 = 600 / 11 ≈ 54.55 km/h.