Exercise 6: Mastering Tens 10-90
Master the tens place! In this exercise, you'll work with all multiples of 10 from 10 to 90. All operands are multiples of 10 (10, 20, 30... 90), and intermediate results stay within 0-90, 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 |
Complement Patterns Across 50
When adding or subtracting with the left hand thumb, observe these patterns:
Addition Patterns:
- Left hand: +40 = +50 -10
- Left hand: +30 = +50 -20
- Left hand: +20 = +50 -30
- Left hand: +10 = +50 -40
Subtraction Patterns:
- Left hand: -40 = -50 +10
- Left hand: -30 = -50 +20
- Left hand: -20 = -50 +30
- Left hand: -10 = -50 +40
⚠️ Important: Start by counting each step individually. Only apply these mental shortcuts once you've become thoroughly familiar with the basic movements!
💡 Tip: For 60-90, remember you're using both your thumb (50) and fingers (10-40). For example: 70 = thumb + 2 fingers. Practice converting back and forth between representations!
Learning Goals
- Master all tens 10-90 on left hand
- Fluent transitions through 50 boundary
- Handle multi-step calculations smoothly
- Build speed and accuracy with tens
Key Concept
Left Hand Values (Tens):
- Thumb = 50
- Each finger = 10
- 60 = Thumb + 1 finger
- 70 = Thumb + 2 fingers
- 80 = Thumb + 3 fingers
- 90 = Thumb + 4 fingers
Remember: Numbers 60-90 always have the thumb extended plus 1-4 fingers.