Strayer Elementary Number Theory 2. 2. 31.

This one's tough. I had to look for help. It took me forever to reach a sensible solution, even with the aid of the (relatively few) tidbits of information online that relate to this problem. I won't source them because they're not big websites or anything. Just the few research papers/ journal articles you've probably already seen in hunting for a solution to this problem. Enjoy.