The sixteen chapters of this book provide the core material for the Elements of Mathematics Program, a secondary sequence developed for highly motivated students with strong verbal abilities. The sequence is based on a functional-relational approach to mathematics teaching, and emphasizes teaching by analysis of real-life situations. This text is intended to provide students with a general background in the analytical methods of mathematics and practice in reasoning with mathematical objects; other texts (Books 1-12) prepared for use with book O explore particular areas in depth. In this chapter, decimals are introduced as common fractions whose denominators are powers of 10. Positional notation, decimal arithmetic, and the approximation of rationals by decimals are discussed. Decimals are applied in the study of percent. (SD)