Definition
Column subtraction
Column subtraction subtracts numbers by lining up equal place values in vertical columns.
What it means
Stack the numbers so matching place values line up. Work one column at a time, usually from the ones. When the top amount is too small, exchange 1 from the next place for 10 of the current place, then subtract.
Remember it
Same lanes, fair trades: 1 hundred = 10 tens, and 1 ten = 10 ones.
Example
Subtract 432 − 178. Exchange 1 ten for 10 ones, then exchange 1 hundred for 10 tens, so 432 is regrouped as 3 hundreds, 12 tens, and 12 ones. Now 12 − 8 = 4, 12 − 7 = 5, and 3 − 1 = 2. The difference is 254.
Watch for
Regrouping changes how the top number is partitioned, not its value: 4 hundreds, 3 tens, 2 ones and 3 hundreds, 12 tens, 12 ones both equal 432.
Common mix-up
Do not subtract the smaller digit from the larger digit regardless of order. Each column is top minus bottom after any regrouping.
Exact meaning
Column subtraction is a written subtraction method that aligns digits with the same place value, subtracts each column, and regroups a higher place when the top amount in a column is too small.