Well polished game with nice art and audio.
The typewriter noise, in particular, is so satisfying.
Only suggestion I would make is trying to make it more obvious to the player that a number can be used more than once (until you pick the modifier where it can't).
I originally assumed that each number *had* to be used exactly once, so I failed the first puzzle with my brain stuck trying to solve it.
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Nathaniel, thanks for playing! I found this great typewriter soundpack on Freesound.org and decided to base the game's whole soundscape around it. Originally, I designed the game to be how you imagined it to be, where each number can only be used once. That was way too hard, so I decided to allow numbers to be re-used and wrote a more gentle solver / puzzle generator. Then when I came up with the modifiers, I realized I could use my original solver again for players who wanted a challenge. Always interesting what players can see no matter how I tried to obscure it.