A box of biscuits contains $36$ biscuits. $20$ biscuits have foil wrappers. $15$ are chocolate biscuits with foil wrappers. If $9$ are not chocolate and do not have a foil wrapper, then how many chocolate biscuits are there? 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle->Inclusion-Exclusion Principle for Two Sets