Two engineering teams, A and B, jointly build a tunnel at the same time. Team A builds 45 meters per day, and Team B builds 55 meters per day, and the repair is completed in 78 days. How many meters is this tunnel?
甲乙两工程队同时合修一条隧道，甲队每天修45米，乙队每天修55米，78天修完．这条隧道长多少米？
<gadget id="calculator">45 + 55</gadget>
<output>100</output>

<gadget id="calculator">100 * 78</gadget>
<output>7_800</output>

<result>7_800</result>