NUMBER EXAM

Once you spot the trick in the question it becomes easy, it's just that without the trick you'll never figure it out, each subsequent line of the code is obtained by writing down how many of each number was in the previous line.

THINK ABOUT IT AS AN EYE EXAM THAT YOU HAVE TO READ EACH LINE OUTLOUD AND THAT CREATES THE FOLLOWING LINE.

So your read out the first line as One One creating the second line 11. dead that out loud as Two One you get the third line 21..

SO we finish with 13112221