A gallery has a collection of $$120$$ paintings, some of which are by French artists. $$40 \%$$ of the paintings are landscapes; the rest are still lifes. Of the landscapes, $$25$$ paintings are by French artists. In total there are $$40$$ paintings by French artists. How many still lifes are there by artists who are~\textbf{not~}French?  Answer:~\uline{~~~~~~~~~~}~ 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle->Inclusion-Exclusion Principle for Two Sets