The distance between A and B is 450 kilometers. The express train and the slow train depart from A and B respectively and travel towards each other. The express train travels 60 kilometers per hour, and the slow train travels 40 kilometers per hour. Let me ask: If the two trains start at the same time, how many hours later will they meet?
甲乙两地相距450千米，快车和慢车分别从甲乙两地出发相向而行，快车每小时行60千米，慢车每小时行40千米，试问：如果两车同时出发，几小时后相遇？
<gadget id="calculator">60 + 40</gadget>
<output>100</output>

<gadget id="calculator">450 / 100</gadget>
<output>9/2 = around 4.5</output>

<result>9/2 = around 4.5</result>