Students interpret simple addition and subtraction situations within 20, identify relevant or missing information, choose an operation, create or interpret stories, and represent relationships with objects, drawings, equations, part-part-whole models, ten-frames, and number lines. They compare numbers primarily within 100, justify comparisons, explain and organize solution strategies, check answers and correct addition or subtraction errors using objects, inverse relationships, representations, and reasonable-size estimates, including straightforward two-digit problems within 100 and number-line reasoning from 0 through 120. They make and test simple conjectures, use logical elimination with small whole numbers and familiar shapes, and communicate known quantities, unknowns, relationships, and mathematical reasoning clearly in words and representations.