数学题:甲乙二人同时从A地出发匀速走向B地，与此同时丙从B地出发匀速走向A地，出发后20分钟甲与丙相遇，相遇后甲立即调头，甲调头后10分钟与乙相遇，然后甲再次调头走向B地，结果当甲走到B地时，乙恰走过A、B两地中点105米，而丙离A地还有315米，求A、B两地的距离。求解。

数学题解答：先画线段图，弄清题中所说的条件。

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