Geometry and Algebra.

Ben Franklin used to do this; if it reminds you of algebra where you "divide both sides by X to cancel out X," that's no coincidence.

Though they may require some basic knowledge of algebra or math skills. 

HP12-C is the ubiquitous one, like how TI-83s are used by algebra students everywhere, same thing. 

Adam, remember when you were in school taking algebra or trig., or some subject that mey have been a challenge and a student would ask "why am I learning this, I am not going to need this for what I am going to be in life!!"?

This is also the investing wisdom from success investors whom I admire.if you get an "A" in Algebra and Biology in high school, but get an "F" in 5 other subjects, your GPA would be horrible and most decent colleges would not accept you, however this is not the case when you investing in real estate or the financial markets. 

I teach s/d all of the time in my general biz courses, but I don't get into the actual algebra.

The banisters will also be custom, and wrap the corner & continue up the wall.We did the stairs with good old fashioned algebra.  

@Jason Hirko: The algebra of this is exactly what I was looking for.

It's just an algebra problem, with dollar signs in front.