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?
Answer Choices
A. 5\:cm
B. 15\:cm
C. 20\:cm
D. 25\:cm
E. 10\:cm