To subtract means to take a number away from another. However, in this case, the number is unknown. This typical of a pre-algebra subtraction exercise on linear equations. This exercise gets a little challenging for beginners and losing interest quickly is not uncommon. To solve this problem, we have made an exciting math game to use for practice. It is the zombie game on pre-algebra subtraction. Click the start button to discover. For fourth grade, fifth grade, sixth grade and seventh grade students.