Topic 1 of 8
Step 3: Adding and subtracting to 20
Adding and subtracting within 20
Step 3 · Adding and subtracting to 20 topics
Add and subtract with numbers up to 20 by counting on, counting back, and making a ten, and use them to solve word problems.
Counting on and counting back
To add a small number, start at the bigger number and count on. 9 + 3: start at 9 and count on three: 10, 11, 12.
To take away a small number, count back. 15 − 2: start at 15 and count back two: 14, 13.
Making a ten
Ten is a handy stopping place. To add 8 + 6, move 2 from the 6 to fill up the 8 to make 10. Then 10 + 4 is easy.
| Step | What you do | Result |
|---|---|---|
| 1 | 8 needs 2 more to make 10 | split 6 into 2 and 4 |
| 2 | 8 + 2 | 10 |
| 3 | 10 + 4 | 14 |
- the 8
- the 6
Subtracting can use ten too. 13 − 4: take 3 to get down to 10, then 1 more to get to 9.
Word problems
Read the story and ask what happens. Joining or getting more means add. Taking away, losing or comparing to find the difference means subtract.
Example: Putting together
Maya has 7 red beads and 8 blue beads. How many beads does she have?
- The beads are put together: add 7 + 8.
- 7 needs 3 more to make 10. Split 8 into 3 and 5.
- 10 + 5 = 15.
Answer15 beads
Example: Taking away
Jon had 16 stickers and gave away 9. How many does he have left?
- Stickers are given away: subtract 16 − 9.
- Take 6 to get down to 10, then 3 more.
- 10 − 3 = 7.
Answer7 stickers
Three numbers can be added two at a time, in any order. In 4 + 7 + 6, add 4 + 6 first to make 10, then 10 + 7 = 17.
Try it yourself
Work each one on paper first, then open the answer to check.
9 + 5
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14. 9 needs 1 more to make 10, then 4 more: 14.
8 + 7
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15. 8 + 2 = 10, then 5 more: 15.
14 − 5
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9. Take 4 to get to 10, then 1 more: 9.
17 − 3
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14. Count back three: 16, 15, 14.
3 + 8 + 7
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18. 3 + 7 = 10, then 10 + 8 = 18.
A shelf has 12 books. 5 are taken. How many are left?
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7 books, because 12 − 5 = 7.