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题目内容
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, 1221 is a palindrome.)
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6.3.1 计数原理
6.3.1 计数原理
以上解析由 考满分老师提供。