This is just a question for fun and specifically for those who have read the reference pervasively.
I have been reading the work of Knuth's The Art of Computer Programming and struggling and having a lot of fun with its exercises. I was very impressed that Fermat's Last Theorem had a difficulty of 45 in Knuth's difficulty scale.
I was wondering, if anyone of you have seen exercises with rank ≥50.
Perhaps a more interesting question, what is the current status of such problems nowadays!?