This was awesome! When I started playing I thought I'd try to get through one or two puzzles to get the idea, but I got hooked and played to the end! I have a whole page of scribbled notes, marking down letters and wondering when and how it's all going to come into play. I liked the math bit, and used a variable as my guess and worked it out with pencil+paper, that was clever. The end gave me a rush as well, such a great feel overall, I loved it.
Amazing work, going to go play the next now! :-)