Really Unsecure Passwords

Discrete Mathematics Level 3

At ReallyUnsecurePassword.com, they demand that users have a password, which consists of the digits from 0 to 9, and is of length \(4\) that uses exactly \(2\) different digits. How many different passwords can there be?

Details and assumptions

\(0001\) is a valid password, while \(3333\) and \(0123\) are not valid passwords because they do not use exactly \(2\) different digits.

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