In order to determine the number of arranges, or permutations, use the following formula:
P = n!/(n - r)!
where n is the number of available letters (n = 5), and r the number of letter for the combinations (r = 3). Replace these values into the previous formula:
P = 5!/(5 - 3)!
P = 5!/2!
P = 1·2·3·4·5/1·2
P = 120/2
P = 60
Hence, there are 60 ways of arranging three letter from a group of five letters, for the security code.
