Two cars, A and B, drive from places A and B at the same time, and meet with car B at a distance of 30 kilometers from place A. After the encounter, the two cars continue to move forward, and then return immediately after arriving at places A and B respectively. On the way, at a distance of 21 kilometers from place B, car A meets car B again. Find the distance between A and B.
甲、乙两辆汽车分别从A、B两地同时相对开出，在甲车离A地30千米处与乙车相遇．相遇后两车继续前进，分别到达A、B两地后又立即返回，途中在离B地21千米处，甲车又与乙车相遇．求A、B两地的距离．
<gadget id="calculator">30 * 3</gadget>
<output>90</output>

<gadget id="calculator">90 - 21</gadget>
<output>69</output>

<result>69</result>