Thank you all for the excellent feedback. I have uploaded the final build for this Trijam, which fixes many issues, but there are still more.
What's been added;
- A new variation of the invaders.
- Ship destruction animation.
- The equation is now easy, medium and hard, with more points for completing the more complex equations.
- I fixed several bugs, including invaders, which will respawn if all are destroyed.
This has been a fun game jam.