先讲一个脑筋急转弯的故事。菏塘里有一片小小的菏叶,它每天会增长一倍,假使30天会长满整个池塘。请问,第28天时这片菏叶会有多大?其实,如果从后往前推,答案立刻就出来了:第28天时这片菏叶会有菏塘的四分之一大。假设第一天开始时你站在菏塘的对岸,会发现那片菏叶是那样小,小得只有一点点。但是,第29天时它就会长成偌大的菏塘的一半,在第30天时就会充满整个菏 Talk about a brain teaser story. There is a small leaf in He Tong, which doubles daily, assuming that the entire pond will be filled in 30 days. Excuse me, on the 28th day, what is the size of this leaf? In fact, if we push forward from the back, the answer will come out immediately: On the 28th day, this quarter will have a quarter of that of He Tong. Suppose you stand on the opposite bank of He Tong at the beginning of the first day and you will find that the leaves are so small and only a tiny bit smaller. However, on the 29th day it will grow to half its size, and on the 30th day it will fill the entire country