其它
In Bill\textquotesingle s class, every student can play either piano, or violin, or both. $24$ of his classmates can play piano, and the number of students that can play violin is half of that of playing piano. If there are $4$ students who can play both, how many students are there in the class? 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle
$24+24\div2-4=32$ 