India, Dec. 9 -- One of the joys of writing this column has been to observe the way readers have solved my puzzles in their own innovative ways, which are often different from the way I would have solved them myself. This has happened most frequently with puzzles involving Diophantine equations, for which we seek integral solutions. The fun part, of course, is that the puzzles in Problematics involve fewer equations and more variables, otherwise they would have been plain textbook exercises.

There are no set rules for how you should solve the following puzzle. There never are. You could use a mix of mathematics and reasoning based on observation, or you could simply go with hit and trial. All that matters is that you send me the correct ...