The math challenge curriculum textbook series is designed to help students learn the fundamental mathematical concepts and practice their in-depth problem solving skills with selected exercise problems. Ideally, these textbooks are used together with Areteem Institute's corresponding courses, either taken as live classes or as self-paced classes. According to the experience levels of the students in mathematics, the following courses are offered: Fun Math Problem Solving for Elementary School (grades 3-5) Algebra Readiness (grade 5; preparing for middle school) Math Challenge I-A Series (grades 6-8; intro to problem solving) Math Challenge I-B Series (grades 6-8; intro to math contests e.g. AMC 8, ZIML Div M) Math Challenge I-C Series (grades 6-8; topics bridging middle and high schools) Math Challenge II-A Series (grades 9] or younger students preparing for AMC 10) Math Challenge II-B Series (grades 9+ or younger students preparing for AMC 12) Math Challenge III Series (preparing for AIME, ZIML Varsity, or equivalent contests) Math Challenge IV Series (Math Olympiad level problem solving) These courses are designed and developed by educational experts and industry professionals to bring real world applications into the STEM education. These programs are ideal for students who wish to win in Math Competitions (AMC, AIME, USAMO, IMO, ARML, MathCounts, Math League, Math Olympiad, ZIML, etc.), Science Fairs (County Science Fairs, State Science Fairs, national programs like Intel Science and Engineering Fair, etc.) and Science Olympiad, or purely want to enrich their academic lives by taking more challenges and developing outstanding analytical, logical thinking and creative problem solving skills.Math Challenge II-A is for students who are preparing for the American Mathematics Competition 10 (AMC 10) contest. Students are required to have fundamental knowledge in Algebra I, Geometry, Basic Number Theory and Counting and Probability up to the 10th grade level. Topics include polynomials, inequalities, special algebraic techniques, triangles and polygons, collinearity and concurrency, vectors and coordinates, numbers and divisibility, modular arithmetic, advanced counting strategies, binomial coefficients, sequence and series, and various other topics and problem solving techniques involved in math contests such as the AMC 10, advanced MathCounts, American Regions Math League (ARML), and Zoom International Math League (ZIML) Division H and Junior Varsity Division. The course is divided into four terms: Summer, covering Algebra Fall, covering Geometry Winter, covering Combinatorics Spring, covering Number Theory The book contains course materials for Math Challenge II-A: Number Theory.We recommend that students take all four terms. Each of the individual terms is self-contained and does not depend on other terms, so they do not need to be taken in order, and students can take single terms if they want to focus on specific topics. Students can sign up for the course at https: //classes.areteem.org for the live online version or the self-paced version.