LR Addition & Subtraction
Work from the left, the way numbers are spoken, and adjust with complements instead of borrowing. No column alignment, no crossing out.
1. The rule
- Left to right: add the hundreds to the hundreds, then the tens, then the units — just as you would read the number out loud. Partial results are always estimates of the answer, never throwaway numbers.
- Complements for addition: turn an awkward addend into a round number. 82 + 49 → 82 + 50 − 1 = 131.
- Complements for subtraction: subtract a round number and give back the difference. 634 − 278 → 634 − 300 + 22 = 356.
- Check with DS before you commit the answer.
One-line version
Add and subtract the big place values first, then repair with the complement. Every intermediate value is a usable estimate.
2. Worked examples
456 + 278
- 456 + 200 = 656
- 656 + 70 = 726
- 726 + 8 = 734
DS: 456 → 15 → 6, 278 → 17 → 8, 6 + 8 = 14 → 5; DS(734) = 14 → 5 ✓
1000 − 456
- Complement of 456 to 1000: 1000 − 400 = 600, − 50 = 550, − 6 = 544
- Check: 456 + 544 = 1000 ✓
3. Practice
- 456 + 278
- 82 + 49
- 1000 − 456
- 634 − 278
- 87 + 68 + 45
Answers
1) 734. 2) 131. 3) 544. 4) 356. 5) 200 (87 + 68 = 155, + 45 = 200).
Previous lesson: Multiplying by 11