問一個數學問題，請大家幫忙，為了孩子考試

來源: 於 2007-01-23 20:59:13 [舊帖] [給我悄悄話] 本文已被閱讀：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