A 120-meter-long train passes through a cave at a speed of 12 meters per second. It takes 70 seconds for the train to enter the cave and exit the cave. How long is the cave?
一列长120米的火车，以每秒12米的速度通过一座山洞，从车头进洞到车尾出洞共用70秒，这座山洞有多长？
<gadget id="calculator">12 * 70</gadget>
<output>840</output>

<gadget id="calculator">840 - 120</gadget>
<output>720</output>

<result>720</result>