I am looking for riddles that are understandable for everyone(so especially non-mathematicians) but require mathematical knowledge or deep abstract ideas to be solved.
The best answer will be the riddle that is most understandable( especially not contain any abstract math at all, so for example fermat's last theorem is NOT what I am looking for) but most mathematically demanding at the same time.
(I hope it is clear how I will try to objectively assess the answers, so that no one has to vote for closing this thread)
Also it would be nice, if the riddle you recommend is not very famous.
Best Answer
Although not needing deep mathematical knowledge...
An old-fashioned implication one:
Robbed these from http://math.alamzy.com/wp-content/uploads/2012/10/Handbook.pdf