Pat and Lee counted leaves on two plants. Pat\textquotesingle s got a $$1$$-digit number. Lee got a $$3$$-digit number. If the difference between their numbers was $$91$$, what was the sum of their numbers? 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle
Pat and Lee counted leaves on two plants. Pat\textquotesingle s got a $$1$$-digit number. Lee got a $$3$$-digit number. If the dīfference of the numbers was $$91$$, the numbers were $$100$$ and $$9$$, and the sum is $$109$$. 