Learning Number pairs to 10

Two numbers that add up to 10 are called different things by different people: number pairs, number bonds, 10 pairs, partitions of 10. Whatever you call them, they are worth memorizing as they help with many other arithmetic problems.

1 + 9

2+ 8

3 + 7

4 + 6

5+ 5

You can see these pairs visually and play with them in my story Ten Little Fish

Using 10 pairs

Adding a 1 digit number to a two digit number:

33 + 8

The ones place of the large number is 3. What is 3's pair? 7. So 33 "needs" 7 more to become 40. It "steals" 7 of the 8 we are adding to do it. So we have 33 + 8 = 40 + 1 = 41.

Or we could have the 8 do the "stealing". 8 needs 2 more to become 10, so 33 + 8 becomes 31 + 10 = 41.

Adding many numbers: finding 10s

If you have a long list of numbers to add, finding groups that make 10 can make it easier to add up.

3 + 8 + 10 + 7 + 5 + 10 + 2 + 5 + 6 =

= 10 + 10 + 10 + 10 +10 + 6 = 56

Subtraction

15 - 8

Think about 15 as 10 + 5. We will take the 8 away from the 10 part because that's easy. We know 8 and 2 are a pair, so 10 - 8 = 2. So after taking away 8 we have 2 + 5 = 7.

When doing multi-digit subtraction with borrowing we do a lot of taking a one-digit number from a "teen" number so this becomes very useful.

342 - 187

Look at the 1s place. Because 7 is more than 2, we will need to borrow.

Now we need to find 12 -7. Think of 12 as 10 + 2, 7 and 3 are pairs to 10, so the answer is 3+ 2 = 5.

Now we borrow from the hundreds place, since 8 is less than 3.

Now we need to find 13-8. 10 - 8 is 2, 13 is 3 more than 10, so 13 - 8 is 2 + 3 = 5.

Now subtract 2-1 in the hundreds place and we are done.

Some people just memorize the subtraction facts like 13 -8. But for those who have trouble memorizing a long list of facts, the 10 pairs can be extended to do a lot of kinds of arithmetic.