There are $$38$$ students in the class. Each student is in either math club or music club or both. Given that among students in this class, $$26$$ students are in the math club, and $$18$$ students are in the music club, there are~\uline{~~~~~~~~~~}~students in the math club only and not in the music club. 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
$$26+18-38=6$$, $$26-6=20$$. 