Question A lock combination code is made up of $4$ numbers ($0-9$). Each number can occur at most twice, e.g. $4764$ would be allowed but not $4464$ as the number $4$ has occurred more than $2$ times.Therefore, how many possible combination codes are there?
I know that there are $10,000$ possible combinations if repetitions are allowed. However I'm unsure as to how to answer the question. Any help is greatly appreciated, thanks!