During the peak of the Spring Festival travel season, if the ticket window is lined up early, people will evenly come to buy tickets one after another. If 5 ticket windows are set up, the queuing phenomenon can be alleviated within 30 minutes. If 6 ticket windows are set up, it will take 20 minutes to ease the queuing phenomenon. Now requires 10 minutes to ease the queuing phenomenon. Q: How many ticket windows should be opened?
春运高峰，售票假设窗口早早地排好了队，陆续还有人均匀的来购票．假如开设5个售票窗口，30分钟可缓解排队现象．如果开设6个售票窗口，那么20分钟才能缓解排队现象．现在要求10分钟缓解排队现象．问：应该开设几个售票窗口？
<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>