and B start from both places A and B at the same time. Walk towards each other. After meeting, continue to move forward in the original direction. When A arrives at B and B arrives at A, they both turn around and return. The two met face to face for the first time at a distance of 20 kilometers from point B, and the second time they met face to face at a distance of 30 kilometers from point B. How many kilometers away from the place where the two met for the third time?
甲、乙两人同时从A、B两地出发．相向而行．相遇后继续按原方向前行，当甲到达B，乙到达A后均掉头返回．两人第一次迎面相遇在离B地20千米处，第二次迎面相遇在离B地30千米处，那么两人第三次相遇地点离B地多少千米？
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<result>10</result>