I am doing my homework after school. When I started writing, I saw that the clock face in the mirror was 6:15. After I finished writing, I looked at the actual clock face and it was still 6:15. How long did it take me to do my homework?
明明放学后写作业，开始写时从镜子里看到钟面是6：15，写完后再看实际钟面还是6：15，明明写作业用了多长时间？
<result>30</result>