題目網址: http://www.puzzleup.com/2019/
http://www.puzzleup.com/2019/puzzle/?01
答題時限: 07月25日７ＰＭ－比賽結束
加分時限: 07月25日７ＰＭ－07月31日６：５９ＰＭ
答對可得基本分１００分。答案可上傳５次，每改１次答案從基本分扣２０分。　
比賽期間內可隨時上傳答案，加分時限內答對第ｎ天加（６－ｎ）分　 　　 　　
另依題目的難易有額外加分（如有８０％的人這題答錯，答對者加８０分） 　　
◆SIX ENVELOPES
Six letters and six credit cards are to be placed into six corresponding
envelopes. However, the letters and the credit cards are placed wrongly,
so that none of the letters and none of the credit cards are placed
correctly.
In how many different ways can this happen?
If the problem was asked for 3 letters, 3 credit cards and 3 envelopes,
the answer would be 4.
六封信件與六張信用卡被放入六個相對應的信封中。然而，信件與信用卡放錯了，
沒半封信件，也沒半張信用卡有放對。
問有多少種不同的放法？
如果題目問的是三封信件，三張信用卡與三個信封的話，答案會是4種。