2007 Fermat Problem 10

A store normally sells each of its DVDs for \$20. At a sale, Phyllis buys two DVDs at the regular price and gets a third DVD for half price. This is the same rate of discount as getting

\textbf{(A)}\ 2 for the price of 1\quad \textbf{(B)}\ 3 for the price of 2\quad \textbf{(C)}\ 4 for the price of 3\quad \textbf{(D)}\ 5 for the price of 4\quad \textbf{(E)}\ 6 for the price of 5

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