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.
Regards,
Nalin Pithwa