From the past, we had Fermat's Last Theorem. Looking for ones that are not solved yet? How about "Collatz conjecture
"? I think there are more complex ones, but yeah, not so simple to explain.
I analyzed the problem for a while and theorized ways to collaboratively contribute to the solution. I settled on submitting an integer sequence to OEIS.
Solving them or progress? May be I don't care, but these little problem, simple ones does encourage one to start thinking about solving problems, they make math accessible to masses.