A and B start from A and B at the same time and walk towards each other with a speed ratio of 7:4. After meeting for the first time, they continue to move forward and return immediately when they reach AB. 10 kilometers, find the distance between A and B.
甲、乙两人从A、B两地同时出发相向而行，速度比为7：4，第一次相遇后继续前进，到达AB两地时立即返回，途中第二次相遇点距A地有10千米，求A、B两地的距离．
<gadget id="calculator">10 / 21</gadget>
<output>10/21 = around 0.47619</output>

<gadget id="calculator">10 / (10/21)</gadget>
<output>21</output>

<result>21</result>