Mental Subtraction Techniques
Being able to subtract quickly in your head is a skill worth building. These techniques help you work smarter, not harder.
1. Compensation Method
Round one number to the nearest ten or hundred, subtract, then adjust.
84 − 39 → round 39 to 40 → 84 − 40 = 44 → add back 1 → 45
2. Bridging Through Tens
Break the subtrahend to reach the nearest ten first, then subtract the rest.
73 − 27 → 73 − 3 = 70 → 70 − 24 = 46
3. Counting Up (Shopkeeper Method)
Instead of subtracting, count forward from the smaller number to the larger.
Find 91 − 67: count from 67 → 70 (3) → 91 (21) → 3 + 21 = 24
4. Breaking Down the Subtrahend
156 − 48 → 156 − 40 − 8 = 116 − 8 = 108
5. Left-to-Right Subtraction
754 − 231 → 700−200=500, 50−30=20, 4−1=3 → 523
Comparison Table
| Technique | Best for |
|---|---|
| Compensation | Numbers near a ten or hundred |
| Bridging | Crossing tens boundaries |
| Counting up | Finding differences (e.g., change) |
| Breaking down | Multi-digit mental subtraction |
Key Takeaways
- Choose the method that suits the numbers involved.
- Compensation works best when one number is just above or below a round number.
- Counting up is great for differences between close numbers.
- Practice these regularly to build confidence and speed.
Practice Questions
- Use compensation: 63 − 28.
- Use bridging: 82 − 35.
- Use counting up: 100 − 63.
- Break down: 245 − 76.
- Left to right: 683 − 251.