Hi all

I am not sure whether this would count as a puzzle but it has puzzled me. I am really bad at combinatorics and need some help. I have stumbled on to the problem which states that

"How many numbers upto N have all unique digits in them?"

eg - suppose N = 34. Then answer is 31 (11, 22, 33 excluded, so 34 - 3 = 31). How do I solve this for a generic case? I cant understand how to extend the idea for 3 digits and above numbers.

Any help would be appreciated. And please do explain your answer to. I would really be thankful for any help which can improve my combinatorics?

Thanks

Anant