Maths Puzzles

I need to do a little more work here, adding some more puzzles and writing up full solutions to those that don't have them. Try your hand at what's here if you like, but there's no prizes other than the knowledge of your success. Where there's a link to a solution, there's also a hint if you just need a helpful tip to get you started. Most of these are either traditional puzzles, or extensions to other people's puzzles. A few are original.