Puzzlers extraordinaire Brian Rabern and Landon Rabern, from the University of California, have kindly shared what they describe as the "simple solution to the hardest logic puzzle ever" in an eight-page answer here.
This is how the completed GCHQ puzzle should look when the correct boxes are shaded in black. However, the puzzle does not end here. The image is a QR code that, when scanned with a smartphone, will take you to the next step. There are five steps in total. However, more than a month after GCHQ set its "fiendishly difficult 'fun' Christmas quiz", not a single person has managed to get all of the answers right, says the Daily Telegraph. The newspaper offers help with the other four steps here.
6. Dot-town suicide riddle
Peter Winkler, professor of mathematics and computer science at Dartmouth College, explains the answer to the Dot-town suicide riddle, along with several other puzzles in a five-page paper here.
