$$30$$ children going to Adventure Park took part in at least one of two events. $$15$$ of them took part in the "moving bridge" contest, and $$20$$ of them went down the zip-wire. How many children from Adventure Park took part in both events? ($$2013$$ Math Kangaroo Problems, Level $$3-4$$, Question \#$$14$$) 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle->Inclusion-Exclusion Principle for Two Sets
$15+20-30=5$ 