25 letters , it is not easy to do. I got this anwser in ten Mts.

来源: 于 2011-03-21 12:26:37 [旧帖] [给我悄悄话] 本文已被阅读：次

I think you can't solve the question about 25 letters. If a child can do it,(s)he would be genius.Please see my proof, it is not easy.We use symmetry and induction,it is not easy for most people.