Donald Duck and Mickey Mouse are running on a circular track. They start from the same place at the same time. Donald Duck runs 3 meters per second, and Mickey Mouse runs 4 meters per second. They run in opposite directions. They meet each other in 45 seconds. How long is the circular runway in meters?
唐老鸭和米老鼠在环形跑道上跑步，两人从同一地点同时出发，唐老鸭每秒跑3米，米老鼠每秒跑4米，反向而行，45秒两人相遇．环形跑道长多少米？
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">7 * 45</gadget>
<output>315</output>

<result>315</result>