2003 Fermat Problem 7

A fish tank, in the shape of a rectangular prism, has a base measuring 20 cm by 40 cm with a height of 30 cm. The tank sits on a horizontal table and is half full of water. If 4000 cm^3 of water is added, what is the new depth of the water?

\textbf{(A)}\ 5 \text{ cm}\quad \textbf{(B)}\ 15 \text{ cm}\quad \textbf{(C)}\ 20 \text{ cm}\quad \textbf{(D)}\ 25 \text{ cm}\quad \textbf{(E)}\ 10 \text{ cm}

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 Fermat contests here.