A and B are practicing race walking on a 400-meter circular track. The two start off at the same time and in the same direction. A walks 300 meters per minute, and B walks 280 meters per minute. After how many minutes, A catches up with B for the first time.
甲乙在400米的环形跑道练习竞走，两人同时同地同向出发，甲每分钟走300米，乙每分钟走280米，多少分钟后甲第一次追上乙．
<gadget id="calculator">300 - 280</gadget>
<output>20</output>

<gadget id="calculator">400 / 20</gadget>
<output>20</output>

<result>20</result>