Question

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

Answer 1

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).