I chose linked lists, and he asked me why not pick arrays. At the time I thought that having the freedom to add and remove digits for addition and subtraction make the linked list more ideal so I mentioned the arrays would cause more overhead than the linked list and described a simple carry ahead adder for base 10. Each node of the linked list represented a digit. The head represented the least significant digit, and increased the importance as you moved through. You'd add each node of the linked list with a carry temp variable and figure out the value to output through modulus and regular division.