2024 Mock AMC 12 - Problem 4

Consider sets \mathcal{A} and \mathcal{B}. Set \mathcal{A} contains 12 elements, exactly 4 of which are also in \mathcal{B}. If the union of \mathcal{A} and \mathcal{B} contains 24 elements, find the number of elements in \mathcal{B} but not \mathcal{A}.

\textbf{(A) }20\qquad\textbf{(B) }12\qquad\textbf{(C) }24\qquad\textbf{(D) }8\qquad\textbf{(E) }0