Riddles, brain puzzles and mathematical problems

[Red]

I have solved it in the past (I think 2 years ago). It's too easy and boring. No thought required, you just have to be patient. I bet Einstein never wrote it. They just gave it this name to popularize the riddle. I don't like it at all. I won't post the solution so others can solve it.

Indeed, i managed to get to that group of 2% too. But i've still seen many struggle with this riddle.