David Dewan came up with this solution (which Richard Lipesalso found on Wikipedia): Let N be any integer not divisible by 2 or 5. Consider repunits R1 = 1, R2 = 11, R3 = 111, , RN+1 and their residues modulo N. There are at most N different residues, so the set of N+1 residues modulo N must contain at least one repeat. Assume RA mod N and RB mod N with B>A are the same. Then (RB RA) mod N = (RB-A10A) mod N = 0. Since N and 10 are relatively prime, N divides RB-A. S/O4. Frank offers this sudoku problem: Many readers tackled it successfully. For anyone who got stuck, heres the answer key: Ten years ago I was doing what is now called AI, Richard Marks 58 writes. He noted that trying all possible iterations of a sudoku problem will tie up your Cray for a week. So I personally wrote the rules and coded this little AI program that solves any sudoko by the time you have released the Solve It button. Since I wrote the program from scratch, I guess you can say I solved S/O4. S/O6. On behalf of the MIT Chess Club, Justin Zhou 25 asked how White can play and mate in two (see below). Frank Model 63 says there are two cases to consider: when Black can castle, and when Black cannot. If Black can castle, then Blacks last move had to be Pc7-c5. In this case White can take en passant, Pb5-c6. If Black castles, then White plays Pb7#. If Black can castle, but makes some other move, then White plays Rf8#. If Black cannot castle, then White plays Ke6. Black cannot escape mate on the next move when White plays Rf8#. Steve Gordon noted that this problem illustrates the three special chess moves. The following was adapted from his analysis with his algebraic chess notation. Black last moved either a king, rook, or c5. If c5, then 1. bxc6 (en passant a.k.a e. p.). If Black can still castle queenside (1. Kc8 & Rd8, a.k.a O-O-O), then 2. b7#. If Black cannot, its king is still trapped on rank 8, so after any black move, 2. Rf8#. If Black last moved a king or rook, en passant is not possible for White, but Black cant castle either, so 1. Ke6 also traps the black king on rank 8, and after any Black move, 2. Rf8#. Note: To create this puzzle, the white bishop on g7 resulted from an underpromotion on h8.