A relation from a set X to a set Y is called a function if each element of X is related to exactly one element in Y. That is, given an element x in X, there is only one element in Y that x is related to. ... This is a function since each element from X is related to only one element in Y.
The key word here is "or". If it's an OR question, you add the probabilities together. If it says "and", you multiply them together As it is an OR question though, you add them. So: 15 + 5 = 20%