A train is 240 meters long. This train travels 15 meters per second. It takes 20 seconds from the time the front of the train enters the cave to when the whole train exits the cave. How long is the cave?
一列火车长240米，这列火车每秒行15米，从车头进山洞到全车出山洞共用20秒，山洞长多少米？
<gadget id="calculator">15 * 20</gadget>
<output>300</output>

<gadget id="calculator">300 - 240</gadget>
<output>60</output>

<result>60</result>