Level 2 · Mental Addition & Subtraction
Left-to-Right Addition
The single biggest upgrade to mental arithmetic: add from left to right, the same direction you read.
The Idea
School teaches column addition from right to left — great for paper, bad for mental math. When you calculate 47 + 32 mentally, it's easier to think "47 plus 30 is 77, plus 2 is 79" rather than "2 plus 7 is 9, carry nothing, 4 plus 3 is 7." Left-to-right mirrors how you read the numbers and keeps your working memory load small.
Worked Example
47 + 32
- Identify the tens part of 32: 30
- 47 + 30 = 77
- Add the units: 77 + 2 = 79
Why This Order?
Adding the larger parts first gives you a close approximation immediately — useful if you only need an estimate. Adding smaller parts last corrects the approximation precisely. By contrast, right-to-left addition gives you an unusable units digit first, then slowly assembles the meaningful leading digits.
Harder Example
689 + 454
- Tens part of 454: 400. 689 + 400 = 1089
- Next: 50. 1089 + 50 = 1139
- Units: 4. 1139 + 4 = 1143
💡 Teaching Tip
Have students narrate their calculation aloud: "I see 47, I add the 30 from 32 and get 77, then I add the 2 and get 79." The verbal narration builds working memory discipline and exposes exactly where students lose track.