陈向东数学机考150题35

huqingjun

A four character password consists of one letter of the alphabet and three different digits between 0 and 9 inclusive. The letter must appear as the second or third character of the password. How many different password are possible？

答案是26*10*9*8*2=37440. 为什么还要乘以2呢？代表什么？

puppycrq

因为字母可能在第二位也有可能在第三位，有两种可能性。