Foundational Numeracy•Subtraction
Mental Subtraction: Using Doubles
Use doubles facts to subtract quickly.
10 min
🎯 What You'll Learn
You will learn to use your knowledge of doubles to subtract quickly in your head.
🏪 Market Story
Emeka needs to solve 16 − 8 quickly. He thinks: "I know 8 + 8 = 16. So 16 − 8 must be 8!" Using doubles makes subtraction fast.
📝 Let's Learn
If you know your doubles for addition, you can use them for subtraction:
- 6 + 6 = 12, so 12 − 6 = 6
- 7 + 7 = 14, so 14 − 7 = 7
- 8 + 8 = 16, so 16 − 8 = 8
- 9 + 9 = 18, so 18 − 9 = 9
- 15 + 15 = 30, so 30 − 15 = 15
When a number is double something, subtracting half gives you back the other half!
Example 1: 14 − 7 = 7. (Because 7 + 7 = 14.)
Example 2: 20 − 10 = 10. (Because 10 + 10 = 20.)
Example 3: 13 − 7 = ? Think: 14 − 7 = 7, so 13 − 7 = 6 (one less).
✏️ Practice Questions
- What is 16 − 8?
- What is 18 − 9?
- ₦50 − ₦25 = ? (Hint: 25 is half of 50!)
Click to see answers
- 16 − 8 = 8. (Double 8 is 16.)
- 18 − 9 = 9. (Double 9 is 18.)
- ₦50 − ₦25 = ₦25. (25 + 25 = 50.)
💡 Remember
If a number is a double, subtracting half gives you the other half. Doubles are powerful for both addition and subtraction!