The distance between A and B is 420 kilometers. Two cars drive from A to B at the same time. The first car travels 42 kilometers per hour, and the second car travels 28 kilometers per hour. A car arrives at point B and returns immediately. How many hours does it take for the two cars to meet each other?
甲、乙两地之间的距离是420千米，两辆汽车同时从甲地开往乙地，第一辆汽车每小时行42千米，第二辆汽车每小时行28千米，第一辆汽车到乙地后立即返回，两辆车从开出到相遇共用多少小时？
<gadget id="calculator">42 + 28</gadget>
<output>70</output>

<gadget id="calculator">2 / 70</gadget>
<output>1/35 = around 0.028571</output>

<gadget id="calculator">420 * (1/35)</gadget>
<output>12</output>

<result>12</result>