Two engineering teams jointly excavated a tunnel, each digging from one end to the middle. The first engineering team excavated 12 meters per day, and the second engineering team excavated 15 meters per day. After 30 days, all of them were opened. How long is this tunnel in meters?
两个工程队共同开凿一条隧道，各从一端向中间开凿．第一工程队每天开凿12米，第二工程队每天开凿15米，经过30天全部开通．这条隧道全长多少米？
<gadget id="calculator">12 + 15</gadget>
<output>27</output>

<gadget id="calculator">27 * 30</gadget>
<output>810</output>

<result>810</result>