How many 3-digit numbers have exactly one digit 5 in them

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How many 3-digit numbers have exactly one digit 5 in them
4月22日11点之前解答,谢谢了!(解释并写出答案）

在所有的三位数中,其中有且只有一位是5的有多少个?

假设5在百位,即：5 _ _ ,剩下的两个位置可以填入0到9（除了5）的任何数,所以他们的组合有9×9＝81个；
若5在十位,即：_ 5 _ ,则百位除5不能填以外,0也不能填,因为要保持是三位数,所以可能的组合有8×9＝72个；
若5在个位,即：_ _ 5 ,则百位和十位都不能填入5和0,所以组合数为8×8＝64个.
综上所述,所有的组合数为81＋72＋64＝217个.

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