What you need to know

Things to remember:

  • We can extend the number line so that we have numbers on the left of 0.
  • We can flip a subtraction, and it will change the answer from positive to negative, or negative to positive.

Usually our number line starts at 0 and goes up in 1s like this:

 

 

And we can use our number line to help us with subtractions.

What is 5 -4?

Using our number line we’d start at 5, and count back 4.

 

 

So, we can see that 5-4=1. But, what would happen if we subtracted a bigger number from a smaller number?

What is 3-6?

 

 

So, we can only subtract 3 before we don’t have anywhere to go, so let’s add our negative numbers.

 

 

Now that we have negative numbers, we can start counting back again.

 

 

So, we can see that 3-6=-3. Interestingly, 6-3=3. Let’s compare these.

 

3-6=-3

6-3=3

 

If we change the order of the subtraction, then we either add or remove the subtraction.

Earlier we saw that 5-4=1, so 4-5=-1. Let’s see some more.

 

8-5=3

5-8=-3

12-11=1

11-12=-1

25-8=17

8-25=-17

Example Questions

Question 1: What is 5-7?

Answer

 

So, 5-7=-2.

 

 

Alternatively, we can flip it!

 

7-5=2

 

so

 

5-7=-2

Question 2: What is 3-8?

Answer

 

So, 3-8=-5.

 

Alternatively, we can flip it!

 

 

8-3=5

 

so

 

3-8=-5