2. So, if I understand the question right, you need to represent each "base 6 digit" by a string of 0's and 1's. Then, for example, a base-6 number with two digits $\displaystyle a_1a_2$ can be written as a binary number $\displaystyle b_1b_2b_3b_4b_5b_6$ where $\displaystyle b_1b_2b_3$ represent $\displaystyle a_1$ and $\displaystyle b_4b_5b_6$ represent $\displaystyle a_2$. Now, the trick is that "fives' complement" of $\displaystyle a_1a_2$ should be represented as $\displaystyle \bar{b}_1\bar{b}_2\bar{b}_3\bar{b}_4\bar{b}_5\bar{ b}_6$ where $\displaystyle \bar{0}=1$ and $\displaystyle \bar{1}=0$.