A four character computer password is generated by choosinf from 26 letters(a,b,. z) and ten digits(0,1,. 9). No password can contain only digits and no password can consist of the same character repeated four times.how many valid passwords are there?

How many total combinations of letters and digits are there? How many digit-only combinations are there? How many passwords are there that are one character repeated 4 times? Is there any overlap between these groups? Are there any other groups that need to be taken into consideration?

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