Regrouping in columns
In the last lesson, each place gave a single digit when adding (less than 10), and when subtracting the top digit in each place was at least as large as the bottom digit. Now other cases appear. If, when adding, the sum in some place is 10 or more, we write only the ones digit of that place and carry the rest to the next higher place. If, when subtracting, the top digit is smaller than the bottom one, we borrow one ten from the next higher place (subtract 1 there) and add 10 in the current place. The order is still the same: ones first, then tens, and so on.
When adding: if the sum in a place is ≥10 ⇒ write the ones digit and carry the rest to the next place
When subtracting: if the top is less than the bottom, borrow 10 from the next higher place
Still line up the columns: ones under ones, tens under tens
Column addition (with carrying)
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