The distance between places A and B is 400 kilometers. Cars A and B drive opposite each other at the same time. Car A travels 38 kilometers per hour, and car B travels 42 kilometers per hour. A swallow travels at 50 kilometers per hour. The speed of meter and car A set off at the same time to fly to car B. When encountering car B, it turns back and flies to car A. It keeps flying like this. How many kilometers does the swallow fly before the two cars can meet?
A、B两地相距400千米，甲、乙两车同时从两地相对开出，甲车每小时行38千米，乙车每小时行行42千米，一只燕子以每小时50千米的速度和甲车同时出发向乙车飞去，遇到乙车又折回向甲车飞去，这样一直飞，燕子飞了多少千米，两车才能相遇？
<gadget id="calculator">38 + 42</gadget>
<output>80</output>

<gadget id="calculator">400 / 80</gadget>
<output>5</output>

<gadget id="calculator">50 * 5</gadget>
<output>250</output>

<result>250</result>