The People's Liberation Army entered the disaster-stricken areas of Sichuan on foot for disaster relief, covering a total of 35 kilometers. In the first 2.5 hours, he walked on flat roads, with an average of 7 kilometers per hour; afterward, he walked on mountainous terrain, with an average of 5 kilometers per hour. How many hours did you walk in the mountains?
解放军徒步进入四川灾区救灾，共行35千米．前2.5小时在平路上行走，平均每小时行7千米；后来在山地行走，平均每小时行5千米．在山地行走了多少小时？
<gadget id="calculator">7 * 2.5</gadget>
<output>17.5</output>

<gadget id="calculator">35 - 17.5</gadget>
<output>17.5</output>

<gadget id="calculator">17.5 / 5</gadget>
<output>3.5</output>

<result>3.5</result>