140

Suppose there are no duplicate letters, number of sequences of a string of 7 letters is 7!

Now, there are 3 duplicates, therefore number of distinct sequences is 7! / 3!

Taking the first four elements yields 7! / 3! / 3! = 140

 

 

所有跟帖： 

• 謝謝小謝。不過這樣解忽略了很多其他排列方式。。。。子女壇那裏基本已經確定答案是２０８。。。：） -塵凡無憂- ♀ (0 bytes) () 10/02/2021 postreply 11:35:14

• 也發覺錯了。 -中間小謝- ♂ (68 bytes) () 10/02/2021 postreply 11:42:06