If a random password is 3 letters(a-z) and 1 number (0-9) what is the possibility that the password will have 2 of the same letters?
26x26x26=17576
There are
26^3 * 10
ways to select characters for the passwords, and
26*2 * 10
ways to select characters for which 2 letters repeat.
Assuming order matters, there are
(4 C 1) 26^3 * 10
possible passwords (we have to choose the position of the number) and
(4 C 1) (3 C 2) 26^2 * 10
passwords in which a letter repeats (we have to choose the position of the number, and which 2 letters repeat).