The length of the train is 400 meters, and it takes 10 minutes to pass through the tunnel (from the locomotive entering the tunnel to the rear of the train leaving the tunnel). If the speed increases by 100 meters per minute, it only takes 6 minutes. Find the length of the tunnel.
火车长为400米，通过隧道(从火车头进入隧道至车尾离开隧道)需10分钟，若每分钟速度增加100米，则只需6分钟．求隧道长．
<gadget id="calculator">100 * 6</gadget>
<output>600</output>

<gadget id="calculator">10 - 6</gadget>
<output>4</output>

<gadget id="calculator">600 / 4</gadget>
<output>150</output>

<gadget id="calculator">150 * 10</gadget>
<output>1_500</output>

<gadget id="calculator">1_500 - 400</gadget>
<output>1_100</output>

<result>1_100</result>