A and B are walking toward each other from two places AB at the same time. When they meet, they are 9 kilometers away from B. A returns immediately after arriving at B, and B also returns immediately after arriving at A. When they meet for the second time, they are 7 kilometers away from A. Ask AB The distance between the two places.
甲乙两人同时从AB两地相向而行，相遇时离B地9千米，甲到B后立即返回，乙到A后也立即返回，第二次相遇时离A地7千米，求AB两地距离．
<gadget id="calculator">3 * 9</gadget>
<output>27</output>

<gadget id="calculator">27 - 7</gadget>
<output>20</output>

<result>20</result>