问一个数学问题，请大家帮忙，为了孩子考试

来源: 于 2007-01-23 20:59:13 [旧帖] [给我悄悄话] 本文已被阅读：次

You are given a collection of 20 multiple choice questions, four each possible answers A, B, C, D, or E. How many different sets of ten questions can be formed if the set has exactly two questions with each possible answer?
answer is 7776, but dont know to how get it
thanks