The two symbols (green square and blue triangle) in the addition above stand for digits (0,1, ...9). It is fairly easy to find out what digits they must be in order that the addition is correct. Use this example to show how algebra, systems of linear equations, can be used to solve this or similar problems. Discuss the role the number of variables and the number of equations play---what additional problems do you get if you have not as many equations as variables.
Erich Prisner, August 2003