Two engineering teams simultaneously dug a 222.6-meter-long tunnel. The first team dug 12.6 meters a day, and the second team dug 15.4 meters a day. The first team started working one day in advance, and the two teams started to cooperate the next day. How many days will it take to open this tunnel?
两个工程队同时相对开凿一条长222.6米的隧道，第一小队每天开凿12.6米，第二小队每天开凿15.4米．第一小队提前1天开始工作，第二天两队开始合作，合作几天可以开通这条隧道？
<gadget id="calculator">222.6 - 12.6</gadget>
<output>210</output>

<gadget id="calculator">12.6 + 15.4</gadget>
<output>28</output>

<gadget id="calculator">210 / 28</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>