A Monopoly Junior Math Equation

One of the many games that Willow enjoys (along with Quirkle, Scrabble, Sorry, chess, checkers, Chutes and Ladders, etc.) is Monopoly Junior:

 

Whenever we play Monopoly Junior, Willow is ALWAYS the banker (she loves it!), and I ALWAYS pay her in large bills and require her to make change. The subtraction itself is, of course, no problem, but Will still needs a lot of practice in the problem-solving aspect--you have to understand, and then memorize, the procedure involved in making change, which can be complicated for a newbie. We verbally go through the procedure each time Will needs to, but I also write it out for her as an equation to reference:

 

And that makes making change much more managable:

 
It's my goal at some point in the near future to translate my chalkboard globe project into a chalkboard game board, which will give me a portable yet large-scale chalkboard so that I can expand the equation into even more mathematical awesomeness, along the lines of "Let P=amount of money give to the banker," etc.