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.
by
Calvin Lin
