Xiaoying and Xiaohong practice running in the same direction on the circular running track. At the start, Xiaoying is 15 meters in front of Xiaohong, Xiaoying runs 4 meters per second, and Xiaohong runs 6 meters per second. How many seconds did it take for Xiaohong to catch up with Xiaoying?
小英和小红在环形跑到上同向练习跑步．起跑时，小英在小红前面15米，小英每秒跑4米，小红每秒跑6米．经过多少秒小红追上小英？
<gadget id="calculator">6 - 4</gadget>
<output>2</output>

<gadget id="calculator">15 / 2</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>