Wow, this is really cool. The idea of a math based combat system is super neat and it's also implemented very well. I enjoyed how each of the enemies had a different type of equation to solve, though it was a little disappointing that the math moth was all the same. The balance is a bit off (I didn't take any damage until the final boss) but that's minor. I'm interested in how this kind of thing could be fleshed out in a larger game. Maybe sometimes you could face multiple enemies at once who combine to make a larger equation?
The sound effects are chosen well and all seem very appropriate for the Gameboy style. The Gameboy green is a great look and the limited colour palette was utilised well. There are some questionable things like rotating pixels but overall it's pretty well done. I will say that some of the numbers were a bit hard to read which made a few equations more difficult than they should be.