The border guards patrolled a total of 26.5 kilometers. In the first 2.5 hours, they walked on flat roads at an average of 5 kilometers per hour. Afterwards, they walked on mountainous terrain with an average of 3.5 kilometers per hour. How many hours did they walk in the mountains?
边防战士巡逻共行26.5千米，前2.5小时在平路上行走平均每小时行5千米，后来在山地行走，平均每小时行3.5千米，在山地行走多少小时？
<gadget id="calculator">5 * 2.5</gadget>
<output>12.5</output>

<gadget id="calculator">26.5 - 12.5</gadget>
<output>14</output>

<gadget id="calculator">14 / 3.5</gadget>
<output>4</output>

<result>4</result>