The book shows some basic ideas on how you can ease the mental arithmetic. By simple multiplication, a handful of simplifications and a system for converting numbers into words (which can be easier to remember) advanced mental arithmetic no longer appear like magic.
One can fairly easily implement the rules and constantly experiencing new feelings of success.
The system for memorizing numbers is somewhat unsuitable for German, because it refers to the letter distribution (and phonetics) of English. One can with little effort but also come up with their own system that better suits the German language.
A lasting impression, however, has not left me the book. Often the next calculator is too close and the PDA stores phone numbers so reliable that one is dependent neither on mental arithmetic even on reminders. We now know, however, that one could do without this helper, if you wanted.
Incidentally, I have brought my English book, because the language is not a problem for me, the German translation should not be so good and the English book is like getting cheaper.
