There are 30 children.  15 of them participated in the "moving bridge" contest.  This means that the other 15, who did not participate in the "moving bridge" surely went down the zip-wire.  In fact 20 went down the zip-wire; therefore, 20-15=5 of the 15 "moving bridges\textquotesingle{} " participants must have also gone down the zip-wire. 
In the Adventure Park, 30 children took part in two of the adventures. 15 of them participated in the "moving bridge" contest, and 20 of them went down the zip-wire. How many of the children took part in both adventures? 

a. $$25$$ 
b. $$15$$ 
c. $$30$$ 
d. $$10$$ 
e. $$5$$ 