A long-distance runner ran a total of 33 kilometers. He ran on flat roads for the first 3 hours, with an average of 5 kilometers per hour. Afterwards, he ran on mountainous terrain, with an average of 3 kilometers per hour. How many hours did he run on the mountainous terrain?
长跑运动员跑步，共跑33千米，前3小时在平路上跑，平均每小时行5千米；后来在山地上跑，平均每小时跑3千米，在山地上跑了多少小时？
<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">33 - 15</gadget>
<output>18</output>

<gadget id="calculator">18 / 3</gadget>
<output>6</output>

<result>6</result>