This textbook is third in a series of arithmetic books that apply the principles discovered by the psychology of learning, by experimental education, and by the observation of successful school practice, to the teaching of arithmetic. The general course, which ends on page 241, is followed by exercises in understanding and using tables and an appendix providing special training for clerical work, farm work, shop work, and further study of mathematics. Certain groups of pupils should be encouraged to study suitable parts of this material in addition to the general course. [For Book Two, see ED623796.]