This section focuses on developing students' understanding and fluency in solving addition and subtraction problems within 20. Students will learn to solve word problems involving various scenarios such as adding to, taking from, putting together, and comparing using objects, drawings, or equations with an unknown. They will also solve problems involving the sum of three whole numbers up to 20. Emphasis is placed on understanding the properties of operations, such as the commutative and associative properties, and applying them as strategies for efficient computation. Subtraction is also framed as an unknown-addend problem to deepen conceptual understanding. Students are expected to develop fluency in addition and subtraction within 10 and apply strategies like counting on, making ten, and decomposing numbers. Additionally, students will explore the meaning of the equal sign, assess the truth of equations, and solve for unknown numbers in equations involving three whole numbers.