Regrouping is the borrowing of a value from one column of numbers to another to aid a mathematical operation. If one is subtracting, it’s necessary to regroup when the number at the top of a column is smaller than the one below it.
When subtracting numbers with more than one digit, write down the larger number first, followed by the smaller number directly below it. For instance, write 15 below when subtracting it from 32. Since 2 is smaller than 5, by regrouping, two becomes 12 when a 10 is borrowed from three on the column to the left. Likewise, 3 loses 1 to become 2. Subtracting 5 from 12 above it on the right column and 1 from 2 on the left column gives 17 as the answer to 32 - 15.