How long would it take someone to solve the calculations to mine a block if they do all calculations with pen and paper (no calculator). Pretend the difficulty is very low and find a share on their first attempt. I think this is very interesting/funny idea

Search Youtube, someone already did this.

They did only 1 hash however that took about 20 minutes by hand.

Which renders it all but impossible. You only get 10 minutes on average, since that's the blocktime. Some blocks solve in lesser time, rarely they are taking more then ten (due to rising difficulty / hashrate).