I am new to combinatorics and might ask a trivial question:
There are $69$ different items, each present $4$ times. From this total of $276$ items, $20$ should be picked at random. I need the formula to find the total number of unique permutations possible. I struggle to deal with the condition that repetitions in the source have a maximum of $4$ and can be combined.
Any help or hint is appreciated.
To make it more down to earth: The $276$ items are lower and upper case characters, numbers and special characters, each present $4$ times. The $20$ equals the password length, formed from this character set. I would like to calculate the password strength against bruteforce attacks.