This book describes the field of fractions, percentages, decimals and proportions. It shows the relations between these topics, and how they can be taught in a way that emphasizes these relations.. The book also describes the need for change in the way we teach mathematics. The authors argue for a shift in emphasis from "acquired skill" to "understanding." First and foremost, students should grasp the underlying concepts. Placing less high demands on the skill level of students in the use of formal procedures can set free time that can be invested in in-depth understanding.