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, the concepts of multiple, divisor and prime are studied. Greatest common divisor and least common multiple are defined; other relationships between numbers and their factors are explored in the text and problems. Many of the problems are applied. (SD)