A train travels from City A to City B, a distance of 300 miles. On its way, it stops at City C, which is exactly halfway between City A and City B. If the train takes 1 hour to travel from City A to City C and 2 hours to travel from City C to City B, what is the train's average speed for the entire journey? Let's think step by step and output the final answer within \boxed{}.
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