A train travels from City A to City B, which are 150 kilometers apart. On its way from City A to City B, the train travels at an average speed of 50 km/h. On its return trip from City B to City A, the train travels at an average speed of 30 km/h. What is the average speed of the train for the entire round trip? Let's think step by step and output the final answer within \boxed{}.
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