Every student in a music group can play either piano or violin, or both. $$30$$ students can play piano; $$41$$ students can play violin, and $$15$$ students can play both. Find the number of students in this group. 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle->Inclusion-Exclusion Principle for Two Sets Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations