The school organized two extracurricular interest groups to go to the countryside. The first group walked 4.5 kilometers per hour, and the second group walked 3.5 kilometers per hour. After the two groups started at the same time for 1 hour, the first group stopped to visit an orchard, spent 1 hour, and then chased the second group. How long will it take to catch up with the second group?
学校组织两个课外兴趣小组去郊外活动．第一小组每小时走4.5千米，第二小组每小时行3.5千米．两组同时出发1小时后，第一小组停下来参观一个果园，用了1小时，再去追第二小组．多长时间能追上第二小组？
<gadget id="calculator">4.5 - 3.5</gadget>
<output>1</output>

<gadget id="calculator">1 / 1</gadget>
<output>1</output>

<gadget id="calculator">3.5 - 1</gadget>
<output>2.5</output>

<result>2.5</result>