The distance between A and B is 840 kilometers. Two cars A and B drive from A and B respectively at the same time. Car A travels 90 kilometers per hour, and car B travels 120 kilometers per hour. After how many hours did the two cars meet?
A、B两地相距840千米，甲、乙两车分别从A、B两地同时相对开出，甲车每小时行90千米，乙车每小时行120千米．经过几小时两车相遇？
<gadget id="calculator">90 + 120</gadget>
<output>210</output>

<gadget id="calculator">840 / 210</gadget>
<output>4</output>

<result>4</result>