Day 9 was by far the easiest day. It was funny because my cousin who is a student teacher currently was talking to me about teaching this exact puzzle to her students. A simpler form but the idea was the same. She was showing them the differences between a pattern and how you can extrapolate that to find the next number.
A little bit of synchroncity there.
This was definite a pretty fast puzzle. Part 2 was hilarious in that it was simply to extrapolate 1 position before the start so it required a minor change to get part 1 to work.