The border guards patrolled a total of 26 kilometers. They walked on flat roads for the first 2.5 hours, with an average of 5 kilometers per hour, and then walked on hills, with an average of 3 kilometers per hour. How many hours have you been walking in the mountains?
边防战士巡逻共行26千米，前2.5小时在平路上行走，平均每小时行5千米，后来在山上行走，平均每小时行3千米．在山地上行走了多少小时？
<gadget id="calculator">5 * 2.5</gadget>
<output>12.5</output>

<gadget id="calculator">26 - 12.5</gadget>
<output>13.5</output>

<gadget id="calculator">13.5 / 3</gadget>
<output>4.5</output>

<result>4.5</result>