How to play Alien Addition on Mac

Alien Addition is an educational game that mixes maths problems with the gameplay of a shoot 'em up. The player controls a cannon left and right along the bottom of the screen and has to shoot down the aliens space ships that appear above him. There are five ships in a row and they slowly begin descending down the screen. Each ship has a maths problem written on it and to shoot it down the player has to enter the correct answer for the ship above him. The player is destroyed if any ship reaches the bottom. The game otherwise runs for a fixed time which can be customized by the player. Other customization options include skill level (1 to 9) and problem range (3, 6 or 9).