Two people, A and B, run counterclockwise on a 400-meter circular track. When starting at the same time, B is in front and A is 100 meters behind B. A runs 5 meters per second, and B runs 4 meters per second. How many seconds does it take for A to catch up with B from the starting line for the second time?
甲、乙两人同在400米的环形跑道上沿逆时针方向跑步，同时起跑时，乙在前，甲在乙后面100米．甲每秒跑5米，乙每秒跑4米，甲从起跑线到第二次追上乙，用了多少秒．
<gadget id="calculator">100 + 400</gadget>
<output>500</output>

<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">500 / 1</gadget>
<output>500</output>

<result>500</result>