This refers to the problems posed in the previous blog HW:
- Let A denote the set for which the set is the same as and B denote the set for which is the same as . A telephone number belongs to the set if and only if all the seven d’s are equal, which can happen in 10 ways only. Hence, . Thus, by the Inclusion-Exclusion Principle, .
- I will post this solution in a few days.