The number of students in Class $6A$ was $\frac{4}{9}$ the number of students in Class $6B$. After some reshuffling, the number of students in both classes are now equal. It was known that $3$ students went from Class $6A$ to Class $6B$ and $18$ students went from Class $6B$ to Class $6A$. Determine the total number of students in Class $6A$ at first. 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle