Hi, 
 this article is a translation of the german wikipedia article Saldenmechanik https://de.wikipedia.org/wiki/Saldenmechanik. 
 I am not an english native and my english sucks. So some of you might help to fix the typos and grammar stuff. 
 I translated this articles because i think this is fundamental knwoledge, relatively easy to understand and because it is of arithmetric nature it is free of economic ideology. 
 It shows basic principles that can be used by everybody, also to reveal common mistakes or even lies of economics or politicians.