Stage 5 of 20
Complement Subtraction
Subtract by thinking about the distance to a friendly ten, then finishing the jump. Complements turn subtraction into two easy steps.
Jump to a ten, then finish
For 13 − 6, jump from 6 up to 10 (that is +4), then from 10 to 13 (+3). Total distance: 7. Counting up is often easier than counting back.
Why this mental math skill matters
Counting up (complement subtraction) often feels easier than counting back, especially when numbers are close. It builds flexible thinking instead of one rigid script.
Worked examples
Study each example slowly. Say the steps aloud — verbal rehearsal strengthens mental arithmetic memory.
15 − 8
- From 8 to 10 is 2.
- From 10 to 15 is 5.
- 2 + 5 = 7.
Answer: 7
12 − 5
- 5 to 10 = 5.
- 10 to 12 = 2.
- Difference = 7.
Answer: 7
21 − 7
- 7 to 10 = 3.
- 10 to 21 = 11.
- 3 + 11 = 14.
Answer: 14
Tips for success
- Start from the smaller number and climb to the larger.
- Use landmark tens as rest stops.
- Verify by adding your answer back to the smaller number.
Common mistakes to avoid
- Mixing up which number to start from.
- Losing the first jump size when computing the second.
How to practice this stage
For each problem, jump to the next ten, then finish to the target. Track both jumps and add them for the difference.
Interactive practice
Stage 5 Drill
Count up from the smaller number to the larger number.
Solve
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FAQ: Complement Subtraction
What is complement subtraction?
It is finding a difference by counting up from the subtrahend to the minuend, often via a friendly ten.
When should kids use counting up?
Whenever numbers are close or crossing a ten makes the path shorter than counting back.