Before the museum opens, there are visitors waiting in line. Assuming that the number of visitors is the same every minute, open 4 doors to let people enter the museum, and there will be no more queuing in 30 minutes. When 5 doors are opened, there will be no more queues in 20 minutes. How many doors need to be opened at the same time if there is no longer a queuing phenomenon in 6 minutes.
博物馆开门前就有参观的观众排队等候，假设每分钟来参观的人数同样多，打开4道门让人们进馆参观，30分钟就不再有排队的现象．打开5道门时，20分钟就不再有排队的现象．如果要在6分钟不再有排队的现象，则需要同时打开多少道门．
<gadget id="calculator">30 * 4</gadget>
<output>120</output>

<gadget id="calculator">20 * 5</gadget>
<output>100</output>

<gadget id="calculator">120 - 100</gadget>
<output>20</output>

<gadget id="calculator">30 - 20</gadget>
<output>10</output>

<gadget id="calculator">20 / 10</gadget>
<output>2</output>

<gadget id="calculator">2 * 30</gadget>
<output>60</output>

<gadget id="calculator">120 - 60</gadget>
<output>60</output>

<gadget id="calculator">2 * 6</gadget>
<output>12</output>

<gadget id="calculator">60 + 12</gadget>
<output>72</output>

<gadget id="calculator">72 / 6</gadget>
<output>12</output>

<result>12</result>