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翟宗荫同志编的《排列和组合》(65年版本)一书中,在关于解“有3封不同的信,投入到4个信箱里,共有多少种投信的方法”?的问题中告诉读者:“若以信箱为主(从信箱收到信)来考虑就非常复杂,而且很难计算出结果来。因此,遇到类似的情况,就应当重新从别的角度来考虑。”笔者认为:“以信箱为主”来解題并不十分难,它可分下列三种情况来考虑。 (一) 四个信箱中某一个信箱收到3封信,也就是从四个信箱中任取一个信箱来接收3封信。这种情况的收信方法可用图示之。
In the book “Arrangements and Combinations” (65th edition) edited by Comrade Zong Zongyin, the readers were asked in the following questions: “There are 3 different letters, put into 4 mailboxes, how many kinds of investment methods are there?” “It is very complicated to consider mailbox (receipt from letterbox) and it is very difficult to calculate the result. Therefore, if you encounter similar situations, you should consider it from another perspective.” The author thinks: “ It is not very difficult to solve the problem based on the mail box. It can be considered in the following three situations. (a) Three mailboxes were received in one of the four mailboxes, that is, one mailbox from four mailboxes was used to receive three letters. The receiving method for this case can be illustrated.