A, B, and C start from the same point on a 400-meter circular runway at the same time, and C is in the opposite direction to A and B. It is known that A's speed is 6 meters per second, B's speed is 5 meters per second, and C's speed is 4.5 meters per second. How many seconds after they set off, C is in the middle of A and B for the first time.
甲乙丙三人同时从400米的环形跑道的同一地点出发，丙与甲乙方向相反．已知甲速度每秒6米，乙速度每秒5米，丙速度每秒4.5米，他们出发后多少秒，丙第一次位于甲，乙的正中间．
<gadget id="calculator">6 + 5</gadget>
<output>11</output>

<gadget id="calculator">11 / 2</gadget>
<output>11/2 = around 5.5</output>

<gadget id="calculator">(11/2) + 4.5</gadget>
<output>10</output>

<gadget id="calculator">400 / 10</gadget>
<output>40</output>

<result>40</result>