Count the number of passwords with following constraints. Assuming digits = {0, 1, …..9},

Question

letters = {a, b, ….. y, z}
(a) 5 characters which are digits or letters.

(b) 4 characters which are digits or letters, with at least 1 digit.
(c) 4 characters which are digits. Adjacent characters are not the same.
(d) 6 characters which are letters. The first half must not equal to the last half.

Answer 1

(a) 36^5
(b) 10 * 36^4 * 5
see what you can do with (c) and (d)