Mental Subtraction: Counting Back
Subtract quickly by counting backward in your head.
🎯 What You'll Learn
You will learn to subtract small numbers by counting backward from the starting number.
🏪 Market Story
Bola needs to quickly work out ₦43 − ₦5 in her head. She starts at 43 and counts back 5: 42, 41, 40, 39, 38. The answer is ₦38! Fast and easy.
📝 Let's Learn
Counting back means starting at the bigger number and counting backward by the smaller number.
This strategy works best when you are subtracting a small number (1 to 9).
Example 1: 56 − 3. Count back from 56: 55, 54, 53.
Example 2: 72 − 4. Count back from 72: 71, 70, 69, 68.
Example 3: 80 − 6. Count back from 80: 79, 78, 77, 76, 75, 74.
Tip: Be careful when crossing a tens boundary! For example, counting back from 31: 30, 29, 28...
✏️ Practice Questions
- 64 − 3 = ? (Count back from 64.)
- 51 − 4 = ? (You'll cross over 50!)
- Amina has ₦85 and spends ₦7. Count back to find how much she has left.
Click to see answers
- 64, 63, 62, 61.
- 51, 50, 49, 48, 47.
- 85, 84, 83, 82, 81, 80, 79, ₦78. Wait — let's recount: 85 − 7. Count back 7: 84, 83, 82, 81, 80, 79, 78. Answer: ₦78.
💡 Remember
Counting back is a fast mental subtraction strategy. Start at the bigger number and count backward by the smaller number. Be careful crossing tens boundaries!