Question 63
Find the maximal cardinality of a set of phone numbers satisfying the following three conditions:
a) all of them are five-digit numbers (the first digit can be $0$);
b) each phone number contains at most two different digits;
c) the deletion of an arbitrary digit in two arbitrary phone numbers (possibly in different positions) does not lead to identical sequences of digits of length $4$.
a) all of them are five-digit numbers (the first digit can be $0$);
b) each phone number contains at most two different digits;
c) the deletion of an arbitrary digit in two arbitrary phone numbers (possibly in different positions) does not lead to identical sequences of digits of length $4$.