标题: prep 94, Help Pls [打印本页] 作者: shanefeng 时间: 2009-12-17 01:06 标题: prep 94, Help Pls 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作者: bingling6251 时间: 2009-12-17 02:57
consist of four different digits。。。 楼主，是different digits哦，所以重复是不行的=。=作者: shanefeng 时间: 2009-12-17 09:36 ... jing ran mei kan dao, diu ren le, bu hao yi
