Invaders Subtraction

Invaders Subtraction is an educational game for the VIC-20.

The player has to defend earth from an alien invasion by doing math. The player controls a gun at the bottom of the screen. From the top, alien saucers will appear and move towards the player. On the saucer is a subtraction problem that the player must solve. The alien is shot down with a correct answer and another appears. Input the wrong number, and the alien will continue towards the player. It's game over when the alien reaches the player. The player wins if they defend earth for two minutes. In harder difficulties the aliens move faster and the problems are harder.