A certain company gives each of its clients a 10 digit number as a sort of identification code. As a precaution, any pair of used codes should differ by at least two digits so no one accidentally gives someone else's code.

How many clients can they have before adding digits? Give an example of a set of codes they might use. What if each pair of codes must differ by at least 3 digits? 4? More?

(In reply to Clarification required by goFish)

Yes, 9999999123 and 9999999132 differ by two digits.  Order does matter.

Posted by Tristan on 2005-11-30 19:05:14