The students had a total of 15 kilometers for military training. In the first 1.5 hours, I walked on flat ground, with an average of 4 kilometers per hour; afterward, I walked in mountains with an average of 2.5 kilometers per hour. How many hours did I walk in the mountains?
同学们进行军训共行15千米．前1.5小时在平地上行走，平均每小时行4千米；后来在山地行走，平均每小时行2.5千米，在山地行走了几小时？
<gadget id="calculator">4 * 1.5</gadget>
<output>6</output>

<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<gadget id="calculator">9 / 2.5</gadget>
<output>3.6</output>

<result>3.6</result>