The distance between the two places is 420 kilometers, A car travels 68 kilometers per hour, and B car travels 72 kilometers per hour. The two cars departed from the two places at the same time and went towards each other. After how many hours did they meet?
两地相距420千米，甲车每小时行68千米，乙车每小时行72千米．两车分别从两地同时出发相向而行，经过几个小时相遇？
<gadget id="calculator">68 + 72</gadget>
<output>140</output>

<gadget id="calculator">420 / 140</gadget>
<output>3</output>

<result>3</result>