2009 Gauss 8 Problem 18

In a class of 40 students, 18 said they liked apple pie, 15 said they liked chocolate cake and 12 said they did not like either. How many students in the class liked both?

\textbf{(A)}\ 15\quad \textbf{(B)}\ 10\quad \textbf{(C)}\ 3\quad \textbf{(D)}\ 7\quad \textbf{(E)}\ 5

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Gauss contests here.