First I use a visual model (like base ten blocks) to explain how regrouping twice works when adding 3-digit numbers. Then we look at how that process works when adding the numbers the usual way (one number written under the other).
We also solve some word problems and puzzles. This lesson is meant for 2nd or 3rd grade math.
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Regrouping in subtraction (3-digit numbers)
Math Mammoth Grade 2 curriculum