题目网址: http://www.puzzleup.com/2019/
http://www.puzzleup.com/2019/puzzle/?01
答题时限: 07月25日７ＰＭ－比赛结束
加分时限: 07月25日７ＰＭ－07月31日６：５９ＰＭ
答对可得基本分１００分。答案可上传５次，每改１次答案从基本分扣２０分。　
比赛期间内可随时上传答案，加分时限内答对第ｎ天加（６－ｎ）分　 　　 　　
另依题目的难易有额外加分（如有８０％的人这题答错，答对者加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.
六封信件与六张信用卡被放入六个相对应的信封中。然而，信件与信用卡放错了，
没半封信件，也没半张信用卡有放对。
问有多少种不同的放法？
如果题目问的是三封信件，三张信用卡与三个信封的话，答案会是4种。