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Grade 1 - Operations and Algebraic Thinking
Standard 1.OA.A.1 - Complete the subtraction sentence by filling in the missing number with operands up to 18.
Use addition and subtraction within 20 to solve word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions, e.g., by using objects, drawings, and equations with a symbol for the unknown number to represent the problem.