Subtraction by regrouping, also known as borrowing, is one strategy for solving subtraction problems. Specifically, regrouping is necessary when subtracting a larger digit from a smaller digit while working from the ones' digit place to the left. To do this, one must regroup, or borrow 10 from the next column.over.
Continue ReadingSubtracting 23 from 87 is fairly straightforward. Lining the digits up into columns by placing the larger number directly above the smaller number and subtracting from right to left, you get 7 - 3 = 4 for the ones' place digit, and 8 - 2 = 6 for the tens' place digit, or 64.
Subtracting 39 from 81 in the same manner, however, poses the problem of how to subtract 9 from 1 in the ones' column. The regrouping strategy provides a solution. Borrowing a 1 from the tens' place digit and moving it to the ones' place results in a 7 plus an 11. Subtracting, 11 - 9 = 2 and 7 - 3 = 4, resulting in the answer to the question, 42.
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