Two engineering teams jointly excavated a tunnel, each working from one end to the other. Team A excavates 4 meters per day, and Team B excavates 3.5 meters per day, and the tunnel is completed in 21 days. How long is this tunnel?
两个工程队共同开凿一条隧道，各从一端相向施工．甲队每天开凿4米，乙队每天开凿3.5米，21天完工，这条隧道长多少米？
<gadget id="calculator">4 + 3.5</gadget>
<output>7.5</output>

<gadget id="calculator">7.5 * 21</gadget>
<output>157.5</output>

<result>157.5</result>