Subtraction by Addition

(also called the Complements Method)

Here we see how to do subtraction using addition!

(I don't recommend this for normal subtraction work, but it is still a valid and interesting way to subtract. And in some cases it may save time.)


Follow these steps:


The "complement" is the number to add to make 10 (or 100, 1000, etc, depending on how many digits we have)

Example The complement of 3 is 7, because 3+7=10 (we add 7 to make 10)

Example: the complement of 85 is 15, because 85+15=100

Example: the complement of 111 is 889, because 111+889=1000

Calculating the Complement

The complement is easy to find!

The basic idea is to find the difference between each digit and 9. That will get us to "999...", so we only need to add 1 to make it "1000..."

In practice it is easy to follow this method:

For the first digit that isn't zero:   find what makes it to 10
For all other digits:   find what makes it to 9

Here are two examples:

(You can check it works by adding the number and its complement,
for example 372+628=1000)

With a bit of experience the "what adds to 10" or "what adds to 9" becomes automatic, and taking the complement becomes quick and easy.

Here is another example where we have to skip over some zeros:

Example: What is the complement of 1700?

So the answer is:


(Check: 1700+8300 = 10000)

Now Add Them!

Now add the two numbers (using column addition), but don't forget to discard the extra "1" on the left.

Here are the 3 steps (complement, add, discard):

And we have found that 653 − 372 = 281 (check it if you want!)

What if the number we are subtracting has fewer digits?

How can we, for example, do 4567 − 56 ?

After taking the complement we just fill any missing spaces with 9s.

Example: 4567 − 56

Well, the complement of 56 is 44 , but we need to "pad it" out to 4 digits, making it 9944. Now we add them:


Then discard the extra "1" on the left, and the answer is:


That case would be easier using Quick Subtraction, but it does show how this "complement, add, discard" method works.

Now, you can practice with these Subtraction Worksheets.