Share your cool maths problems.

Complete a challenge:

• Post your solution in comments, if it is exactly the same as OP's solution, let us know.
• Have fun.

Post a challenge:

• Doesn't have to be original, as long as it is not a duplicate.
• Challenges not riddles, if the post is longer than 3 paragraphs, reconsider yourself.
• Optionally include solution in comments, let it be clear this is not a homework help forums.
• Tag [unsolved] if you don't have a solution yet.
• Please include images, if your question includes complex symbols, attach a render of the maths.

Feel free to contribute to a series by DMing the OP, or start your own challenge series.