1.NBT.C.4.a

Use concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a number sentence; justify the reasoning used with a written explanation.

Khan Academy Resources

Subtract within 100 using place value blocksAdd within 100 using place value blocksAdd within 100 using a number lineAdding up to four 2-digit numbersSelect strategies for adding within 100Break apart 2-digit addition problemsSubtract within 100 using a number lineAdd 2-digit numbers by making tensAdding by making a group of 10Missing numbers in addition and subtractionAddition and subtraction with number linesAdding 2-digit numbers without regroupingSubtracting 1s using place valueSubtracting a 1-digit number with regroupingSubtracting 10s using place valueUnderstanding place value when adding onesStrategies for adding 2-digit numbersSubtracting two-digit numbers without regroupingAdding with regroupingUnderstanding place value when adding tensBreaking apart 2-digit addition problemsAdding 53+17 by making a group of 10

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