2000 Fermat Problem 3

A farmer has 7 cows, 8 sheep and 6 goats. How many more goats should be bought so that half of her animals will be goats?

\textbf{(A)}\ 18\quad \textbf{(B)}\ 15\quad \textbf{(C)}\ 21\quad \textbf{(D)}\ 9\quad \textbf{(E)}\ 6

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