prep 94, Help Pls

shanefeng · 发表于 2009-12-17 01:06:54

A company plans to assign identification numbers to its employees. Each number is to consist of four different digits from 0 to 9, inclusive, except that the first digit cannot be 0. How many different identification numbers are possible?
(A) 3,024
(B) 4,536
(C) 5,040
(D) 9,000
(E) 10,000

I choose D.
here is the solution from prep notes
Each number is to consist of four different digits 為四位數
the first digit cannot be 0 故有1到9 九個數字
the SECOND digit 可以為0但是因為the first digit用到一個數字所以剩下九個數字
the THIRD digit 剩下八個數字
the fourth digit 剩下七個數字
9×9×8×7=4536

however, I think the number on each digit can be repeated. For example 2222, 1111 are valid ID number. So it should be 9*10*10*10=9000

bingling6251 · 发表于 2009-12-17 02:57:21

consist of four different digits。。。
楼主，是different digits哦，所以重复是不行的=。=

shanefeng · 发表于 2009-12-17 09:36:30

... jing ran mei kan dao, diu ren le, bu hao yi

prep 94, Help Pls

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