The lotus leaves in the pond double in size every day. On the 30th day, the lotus leaves cover the whole lotus pond. Then how many days do the lotus leaves cover a quarter of the whole pond?
池塘里的荷叶每天长大一倍，第30天荷叶将整个荷塘覆盖，那么第多少天荷叶覆盖整个池塘的四分之一．
<result>28</result>