A and B set off from A and B at the same speed at the same time and walk toward each other. The place where they meet for the first time is 7 kilometers away from A. After meeting, they continue to move forward and return immediately after reaching the starting point of the other party. , met for the second time at a distance of 3 kilometers from point B, how many kilometers did they walk when they met for the third time?
甲、乙二人以均匀的速度分别从A，B两地同时出发，相向而行，他们第一次相遇地点离A地7千米，相遇后二人继续前进，走到对方出发点后立即返回，在距B地3千米处第二次相遇，求第三次相遇时共走了多少千米？
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 - 3</gadget>
<output>18</output>

<gadget id="calculator">1 + 2 + 2</gadget>
<output>5</output>

<gadget id="calculator">18 * 5</gadget>
<output>90</output>

<result>90</result>