2020 AMC 12A Problem 1

Carlos took 70\% of a whole pie. Maria took one third of the remainder. What portion of the whole pie was left?

\textbf{(A)}\ 10\%\qquad\textbf{(B)}\ 15\%\qquad\textbf{(C)}\ 20\%\qquad\textbf{(D)}\ 30\%\qquad\textbf{(E)}\ 35\%

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.