Amazon.com description: Product Description: Algebra is a foundational concept in the study of mathematics, and readers will delight in learning the principles of operations and algebraic thinking in a relatable and engaging way. Topics such as adding and subtracting within 20, using algebraic principles to solve equations and word problems, and understanding the relationship between addition and subtraction will give beginning math students a firm grasp on basic math concepts. Readers will practice subtracting within 20 at a book sale, where books flying off the shelves become the factors in subtraction equations. This volume satisfies CCSS Math Standard 1.OA.A.1.