Different people find different things hard. So a problem that is hard for someone may seem easy to you and one that you think is hard may be easy for someone else. It is, therefore, not possible to answer the question.
i
I am pretty sure you can make them as hard as you want - if you have a "hard" problem, you can always find one that is even harder. If this is true, then there is no such thing as a "hardest algebra problem".
Immortality.. I supose.
900x10080
Proving the Riemann conjecture.
That's hard to say.
Anyone can if they work hard at it.
stating the problem and then designing the experiment.
Depends whats hard for you.
you Crazy! It's pi!( the number of times that diameter fits around the circumference) it still hasn't been fully defined yet.
Adobe
you can die up to 5000 times