# Column Subtraction

Column subtraction is very useful if you need to subtract one number from another and you are unable to use mental subtraction

For example:

35 - 24

To do column subtraction correctly it is important that you understand place value and put each number in the correct column

First you need to put the smaller number underneath the larger number so that the tens are above the tens and the units are above the units.

Take the bottom number from the top number in the units column first. 5 - 4 = 1 so the number 1 is placed below the units.

Next take the bottom number from the top number in the tens column. 3 - 2 = 1 so the number 1 is placed below the tens.

The answer is 1 ten and 1 unit or 11

If the top digit in any of the columns is smaller than the bottom digit then we have to borrow from the next column.

In this example the digit 5 at the top of the units column is smaller than the digit 8 below.

It is not possible to take 8 away from 5 without leaving a negative number so we borrow 1 ten from the tens column and add it to the 5 making 15.

Because we have borrowed 10 from the tens column we need to take 1 ten away from that column changing the 4 to a 3

Having borrowed 1 ten we can work out the units column. 15 - 8 = 7 so we put the answer 7 for the units column.

Next we work out the tens column using the new number. 3 - 2 = 1 so we put the answer 1 for the tens column.

The answer is 45 - 28 = 17

The same method is used to subtract bigger numbers

The numbers in the units column are calculated first. The digit 3 is smaller than the 6 below so it is necessary to borrow 1 ten from the tens column. The 3 in the units column becomes 13 while the 4 tens in the tens column are reduced to 3.

The units column can be worked out 13 - 6 = 7 so a 7 is placed as the answer for the units column.

In the tens column the top digit is smaller than the bottom digit so we have to borrow 1 ten from the hundreds column. We borrow 1 ten because there are 10 tens in a hundred. (10 x 10 = 100). The 3 in the tens column becomes 13 while the 8 in the hundreds column becomes 7.

The tens column can be worked out 13 - 5 = 8 so an 8 is placed as the answer for the tens column.

The 7 in the hundreds column is larger than the 5 so that column can be worked out 7 - 5 = 2 so a 2 is placed as the answer for the hundreds column.

The answer to the sum 843 - 556 = 287