The distance between A and B is 432 kilometers. There are three people A, B and C. A and B start from A and C start from B at the same time and walk toward each other. It is known that A travels 36 kilometers per hour and B travels per hour. 30 kilometers, C travels 24 kilometers per hour, how many hours later, B is exactly at the midpoint between A and C?
A、B两地相距432千米，有甲、乙、丙三人，甲、乙从A地，丙从B地同时出发相向而行，已知甲每小时行36千米，乙每小时行30千米，丙每小时行24千米，问几小时后，乙正好在甲、丙两人的中点？
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