There were $30$ students in a line facing their teacher. The teacher instructed them to call out the numbers $1$ , $2$ , $3$ , $\cdots$ , $29$ , $30$~ from left to right, asking those who have called out a multiple of $2$ to turn around. Then she asked those who have called out a multiple of $3$ to turn around. How many students ended up facing the teacher? 
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