Exercise 5: Working with 50 (Tens Place)
Now it's time to master the left thumb (value 50)! Every problem in this exercise includes the number 50 either in the calculation or result. All intermediate and final results stay within 10–50, using your left hand only.
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Show the result on your left hand
Conversion Technique: Thumb and Other Fingers (Tens Place)
Basic Rules (Left Hand):
- Extended finger = counting
- Bent finger = not counting
- 4 fingers (index, middle, ring, pinky) = 10 each
- Thumb = 50
Key Principles:
- When thumb is extended and want to subtract ten → thumb bends, 4 other fingers extend
- When 4 fingers are extended and want to add ten → thumb extends, 4 other fingers bend
Example: 20 + 30 = 50
Step 1: Extend 2 fingers (index + middle) = 20
Step 2: Add 30 means need to extend 3 more fingers
- Extend ring finger = 30
- Extend pinky = 40
- Out of fingers! Need 1 more finger
Step 3: Conversion
- Bend all 4 fingers
- Extend thumb → 50!
Explanation: 20 + 30 = 50 is actually 20 + 20 = 40, then 40 + 10 = 50 (convert 4 fingers to thumb)
Example: 50 - 20 = 30
Step 1: Extend thumb = 50
Step 2: Subtract 20 means need to reduce by 2 units (2 fingers)
- Need to subtract 10 first: Bend thumb
- But no other fingers extended to bend!
Step 3: Conversion
- Bend thumb
- Extend all 4 fingers = 40
- Continue subtracting:
40 - 10 = 30→ Bend pinky
Result: 3 fingers extended (index + middle + ring) = 30
Explanation: 50 - 20 = 30 is actually 50 - 10 = 40 (convert thumb to 4 fingers), then 40 - 10 = 30
Quick Conversion Reference:
| Calculation | Action |
|---|---|
40 + 10 = 50 | Bend 4 fingers → extend thumb |
50 - 10 = 40 | Bend thumb → extend 4 fingers |
30 + 20 = 50 | Extend 1 more (= 40), then convert to 50 |
20 + 30 = 50 | Extend 2 more (= 40), then convert to 50 |
50 - 20 = 30 | Convert to 40, then bend 1 finger |
💡 Tip: Practice the conversion slowly at first. The key is understanding that 4 fingers (40) + 10 = thumb (50) and thumb (50) - 10 = 4 fingers (40). Speed comes with repetition!
Learning Goals
- Master using the left thumb for 50
- Understand conversion between 40 ↔ 50
- Smooth transitions in multi-step problems
- Build foundation for numbers 60-90
Key Concept
Left Hand Values (Tens):
- Thumb = 50
- Index finger = 10
- Middle finger = 10
- Ring finger = 10
- Pinky = 10
Remember: When thumb is extended, the 4 fingers are bent. When 4 fingers are extended, thumb is bent.