What is this skill?
Subtracting within 20 covers problems such as 13 − 8, 17 − 9 or 12 − 3. First graders are expected to be quick with subtraction within 10 and to use strategies for the bigger problems instead of counting back one at a time.
The most powerful strategy is thinking addition. To solve 13 − 8, a child asks "8 plus what makes 13?" and, knowing 8 + 5 = 13, answers 5. Another strategy is going through ten: for 13 − 8, take away 3 to get to 10, then take away the remaining 5 to reach 5. For small subtractions like 15 − 2, counting back still works well.
Why it matters
Subtraction facts are needed constantly in later grades: in two-digit subtraction, telling time, money and measurement. More importantly, learning to see subtraction as the opposite of addition is one of the central ideas of first grade math. It means a child knows a subtraction fact whenever they know the matching addition fact, which halves the memory work.
Going through ten is also the mental version of the regrouping children will learn for written subtraction in second grade, so practicing it now with small numbers makes that later step far easier.
Common mistakes
- Counting back from the first number and including it: for 13 − 8, saying "thirteen, twelve, eleven..." and landing one too high. Counting back works best for small subtractions; for larger ones, counting up from 8 to 13 is easier and more accurate.
- Subtracting the smaller digit from the larger one, so 12 − 7 becomes 15: the child sees 2 and 7 in the ones, works out 7 − 2 = 5, and keeps the 1. Asking "7 plus what makes 12?" avoids the trap and gives 5.
- Not connecting the two operations, so every subtraction is solved from scratch. After each subtraction, ask for the matching addition fact to build the link.
How to help at home
- Use fact families. Write three numbers, such as 6, 8 and 14, and ask your child to make two addition and two subtraction facts with them.
- Play "how many more to get there?": "You have 9 stickers. How many more until you have 15?" This is subtraction disguised as counting up, and children often find it easier.
- Draw a number line from 0 to 20 and show a subtraction as a jump from the smaller number up to the bigger one. The length of the jump is the answer.
- Use everyday comparisons: "Your tower has 16 blocks and mine has 9. How many more does yours have?"
Worked examples
These are real questions from the Tutorsaurus subtracting within 20 practice, with the answers worked out.
Example 1
10 − 3 = ?
Answer: 7. 10 − 3 = 7. Think: 3 + ? = 10.
Example 2
20 − 3 = ?
Answer: 17. 20 − 3 = 17. Think: 3 + ? = 20.
Example 3
20 − 2 = ?
Answer: 18. 20 − 2 = 18. Think: 2 + ? = 20.