reason :

來源: 於 2010-06-21 13:14:17 [舊帖] [給我悄悄話] 本文已被閱讀：次

why 70 :

Total number of wrong answers from all people is 90. For number of people having 3 or more correct answers to be minimal, the number of people making 3 or more mistakes should be maximal. So, these 90 mistakes should be made by 30 people, each of them makes 3 mistakes.