数学题：早上7点，甲乙两人从A、B两地同时出发相向而行，甲每小时比乙每小时多行x千米，x为整数，在距离AB的中点10千米处，甲与乙第一次相遇，之后乙继续前行于下午到达A地等待甲回来，甲到达B地后立即返还，于下午5:00到达A地，问乙等了甲多长时间？

数学题解答：

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