There are $$30$$ children in Mrs Patel's Year $$6$$ class.  $$8$$ of them are wearing glasses.  $$12$$ of them are wearing a watch.  $$7$$ of them are wearing both glasses and a watch.  How many are wearing neither glasses nor a watch? 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle->Inclusion-Exclusion Principle for Two Sets