Left-to-Right Addition: Big Numbers First
Level 1 Mental addition · Reviewed by The NumThink editorial team · Last reviewed
This page is part of the NumThink Mental addition course. Read why the method works, watch it run on worked examples, then drill it in the practice box below until it feels automatic.
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No signup · instant feedbackWhy it works
Paper addition goes right-to-left because carrying is easier to write than to remember. Mentally, the order you process digits is free — and left-to-right means you hold a shrinking number in your head: 600, then 730, then 743.
You also get a running estimate: after the first step you already know the answer is "six-hundred-something". That estimate catches gross errors instantly.
The steps
- Add the biggest parts. 476 + 267: hundreds first — 400 + 200 = 600.
- Add the next parts. Tens: 70 + 60 = 130. Running total: 600 + 130 = 730.
- Finish with the smallest parts. Ones: 6 + 7 = 13. Total: 730 + 13 = 743.
Worked examples
Easy 34 + 52
Show the thinking
30 + 50 = 80
4 + 2 = 6
80 + 6 = 86.
Answer: 86
Medium 476 + 267
Show the thinking
400 + 200 = 600
70 + 60 = 130 → 730
6 + 7 = 13 → 743.
Answer: 743
Hard 3,452 + 1,837
Show the thinking
3,000 + 1,000 = 4,000
400 + 800 = 1,200 → 5,200
50 + 30 = 80 → 5,280
2 + 7 = 9 → 5,289.
Answer: 5,289
When to use it
- Any multi-digit addition — it is the default mental method.
- Estimating: the first partial sum is already a usable estimate.
When NOT to use it
- Single-digit facts you know instantly (7 + 8) — recall beats procedure.
- Very unbalanced sizes (4,000 + 17) — just append the small part.