Today, I added the final few levels, all the way from 14 to 20, added the final final level (with credits), and made the whole T and F equations much easier to understand thanks to a better tutorial level! You can thank me later
Besides that, I've gone through all 20 levels, and picked up which ones are eligible for a secret stage, and apparently four of them are, so I went ahead and made 4 secret levels, two of which are harder versions of the previous puzzle, one that just teaches you the mechanics of secret levels, and one that is just a joke. Seriously, it may even be easier than the stage it comes from.
An undo button was created, attempts at animating a level-to-level transition (key word being attempts), polished some more graphics, added a different background for secret levels, and that's basically it.
Tomorrow is the last day, so I'll add some music from Kevin MacLeod (incredible music and all royalty free!), add a level menu, try to add transitions from level to level, and add a couple of achievements. Oh, and the game will automatically keep track of the levels you've beat.
Apart from the transitions, I think I'll manage all of the above!