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A certain personal identification number (PIN) for opening garage doors is a sequence of $$4$$ digits, where each digit is selected from the digits $$0$$ to $$9$$. How many such PINs have at least two different digits and are palindromes?
(Note: A palindrome is a sequence of characters that reads the same backward and forward. For example, $$1,221$$ is a palindrome.)
(Note: A palindrome is a sequence of characters that reads the same backward and forward. For example, $$1,221$$ is a palindrome.)
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· 相关考点
6.3.2 乘法原理
6.3.2 乘法原理
以上解析由 考满分老师提供。
