playground has 400 people waiting in line before the gate opens. After the gate opens, the number of visitors per minute is the same. One entrance can accommodate 10 tourists per minute. If 4 entrances are opened, there will be no one after 20 minutes. Queue, now that 6 entrances are open, no one will line up for a few minutes after the door is opened.
某游乐场在开门前已经有400个人在排队等候，开门后每分钟来的游客人数是相同的，一个入口处每分钟可以放入10名游客，如果开放4个入口，20分钟后就没人排队，现在开放6个入口处，那么开门后多少分钟就没有人排队．
<gadget id="calculator">6 * 10</gadget>
<output>60</output>

<gadget id="calculator">20 * 10 * 4</gadget>
<output>800</output>

<gadget id="calculator">800 - 400</gadget>
<output>400</output>

<gadget id="calculator">400 / 20</gadget>
<output>20</output>

<gadget id="calculator">60 - 20</gadget>
<output>40</output>

<gadget id="calculator">400 / 40</gadget>
<output>10</output>

<result>10</result>