I'm neither interested in squaring three-digit numbers in my head nor in doing mathematical magic. I'm only interested in practical mental math. I did buy this book for chapter 5; *** Good Enough; The Art of 'Guesstimation' ***. I've been doing guesstimations for many years intuitively, not very structured. This fine chapter teaches beginners the art of guesstimation while experts can go back to the basics and hone their skills while they might discover something new ... I did do so with approximating years needed for trippling an investment given interest rates of X% p.a.