AMC 8 1993 Problem 17

Square corners, 5 units on a side, are removed from a $20$ unit by $30$ unit rectangular sheet of cardboard. The sides are then folded to form an open box. The surface area, in square units, of the interior of the box is

$\text{(A)}\ 300 \text{(B)}\ 500 \text{(C)}\ 550 \text{(D)}\ 600 \text{(E)}\ 1000$