### In this lesson:

- Why numbers like 10, 20, 30… are very useful
- How to use these numbers in addition
- Reversing the order of numbers

## Number Pairs

It is important to know the five pairs of numbers that add up to 10. These are:

1 + 9 = 10

2 + 8 = 10

3 + 7 = 10

4 + 6 = 10

5 + 5 = 10

## Multiples of Ten

Numbers like 10, 20, 30, 40 and so on, are very useful.

We call the numbers 10, 20, 30, 40, 50… **multiples of 10**, because they start at 10 and go up in 10s.

Now think of the number 39.

You can see that 39 is close to 40 and is 1 short of 40.

And you can also see that 58 is close to 60 and is 2 short of 60.

This makes it easier to work with numbers, adding and subtracting them.

## Addition in Action

Suppose you want 38 + 5.

You know that 38 is close to 40 and is 2 below it.

So take 2 of the 5 to make up to 40 and you have 3 more to add on, which gives 43.

So 38 + 5 = 43.

## Order of Addition

Notice also that if you wanted, say 5 + 77, you could add 5 to 77 because *5 + 77* will give the same answer as *77 + 5*.

The order the numbers are in doesnâ€™t matter, you can always reverse the order if you want to.

So 5 + 77 = 82.